...there's plenty of related info available straight from the Mouse's mouth, with opportunities for reasonable speculation. The following opinions stem from study of RCID documents, news articles, observation of Disney's past decisions, and logical extrapolation therefrom--with the full realization that nothing is set in stone.
Summary: There are multiple options for possible new resort sites available for Disney to build on without using wetland offsets. They probably aren't in a hurry to build in the near future anyway.
First, Disney's current game plan for new lodgings appears to focus upon existing flag-brand partners who build their hotels on land purchased (as opposed to ground-leased) from Disney, as with Four Seasons and at Flamingo Crossings.
Meanwhile, the never-built Wilderness/Buffalo Junction site between the Lodge and the Fort is the only large area planned for Hotel/Resort use that has yet to see resort construction. Disney has a clear tradition of repurpose/reuse, as with Art of Animation, which was the Epcot Center Ultralight Airpark before it was Pop Century: Legendary Years. It's a reasonable guess that the Junction will one day become the site of an official WDW resort.
On the other hand, the never-built Persian resort site has been a candidate resort location at least as recently as 1991. Situated off World Drive northeast of Space Mountain, the parcel is designated as Existing Development, indicating that its suitability is proven. The adjacent monorail line, once planned as a Resort Loop tie-in, would most likely retain its current function solely as access to the maintenance shop nearby. The area is far enough backstage that it's most likely earmarked for expansion of adjacent facilities or continuation of its current use as a staging area and CM-only parking lot, which might have substantial logistical value to Disney. It would be a considerable surprise to see a new resort go in here.
Beyond the currently designated Hotel/Resort and Existing Development areas...if Disney ever decides to undertake construction of new resorts in land areas designated for Mixed Use, RCID identifies many suitable options--several of which present intriguing challenges. There would be little if any need for Disney to offset use of marginally suitable wetland acreage if it chose to build on its internal suitable Mixed Use parcels.
Below is a list of areas within WDW that are designated as Suitable
and as Mixed Use, which RCID defines as "innovative combinations of land use and development intensity." Source: RCID Comprehensive Plan 2020 (see Figures 2-1, 2-5, 2-6, 3-3, and 3-10).
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1.) AIRFIELD ENVIRONS: This very large parcel surrounding the former "Lake Buena Vista STOLport" airfield, between World Drive and Fort Wilderness, has plenty of resort space with existing road access via Vista Blvd and World Drive. The airstrip has not been used as such in some years, and it's currently a maintenance staging area. The parcel could easily hold a new resort with extensive associated recreational facilities. RCID shows the area as Potential Hotel/Resort/Commercial/Entertainment space, and it's prominently mentioned in their discussion of suitable land. This is arguably the most likely site for new resort construction in a Mixed Use area.
2.) STUDIOS WEST: This parcel west of HS, on the opposite side of World Drive, has sufficient space for a new resort--even consideering the contiguous canal and Marginally Suitable/Resource Management areas. Access via Buena Vista Drive appears to be already planned across from the Coronado Springs entrance. RCID indicates that the area is Potential Hotel/Resort space, and it's prominently mentioned in their discussion of suitable land. It's very likely that Disney will eventually undertake new resort construction here.
3.) ESPN NORTH: This very sizeable parcel has plenty of space for a resort and some sports facilities connecting to the existing ESPN complex, with an entry road already traced in from Victory Way. RCID indicates that the area is Potential Hotel/Resort space. A reason to guess otherwise is the stated purpose of the current and planned Flamingo Crossings hotels, which target the sports-tourism sector focused on ESPN. A resort here might be at cross purposes with that plan. Even so, it's large, available, and prominently mentioned in RCID's discussion of suitable land, making it a likely choice for future resort construction.
4.) OSCEOLA SOUTH: This site's acreage approximates the overall footprint of Wilderness Lodge, so a new resort could fit. An access road (Palmetto Place, off Victory Way) already exists, continuing back to the recently built laundry facility across Osceola Parkway from Pop Century. RCID considers the area (as well as the laundry facility) Potential Hotel/Resort/Commercial/Entertainment space, and it's prominently mentioned in their discussion of suitable land. But such use might conflict with the planned Flamingo Crossings sports-tourism focus, as with "ESPN North" above. It's hard to guess what Disney might build here, but there's a fair chance that eventually a resort could go in.
5.) VISTA SOUTH: This large parcel south of Fort Wilderness, on the opposite side of Vista Blvd, has much to recommend as a resort site. It's big enough and offers easy road access, with a good-sized natural area to inspire landscaping opportunities. If there's a downside to building anything here, it's not apparent. It's unclear why RCID doesn't show it as a Potential Hotel/Resort space, nor is it mentioned in their discussion of suitable land.
6.) MAGNOLIA NORTH: The parcel north of the Magnolia/Oak Trail golf complex is large enough for a resort, even though it's laced with smaller Marginally Suitable/Resource Management/Recreation areas. The likeliest path for road access from any resort buildings to Floridian Way is tight but manageable. The site is close enough to the monorail line that it would raise the question of a new stop on the Resort Loop, which would present several challenges. First, the extra stop between Grand Floridian and MK would likely diminish the monorail experience for guests eager to reach MK. Next, the logical site for a collocated monorail station is just west of the inlet where Electrical Water Pageant boats emerge into Seven Seas Lagoon; the prospect of new station construction (and huge walls) without disrupting current operations is daunting at best. Further, based on the configuration of the parcel, pedestrians would need to cover inconvenient distances and cross Floridian Way to reach the station. It's hard to see how Disney could make all that work well. Another golf course (or a golf-focused resort with bus-only transportation, like Shades of Green) would better fit the terrain but hardly seems compelling for a new Disney story. Despite the many inherent constraints here, the area is prominently mentioned in RCID's discussion of suitable land. On the other hand, RCID doesn't show the area as a Potential Hotel/Resort space.
7.) SEVEN SEAS EAST: The site of the once-planned, since-abandoned Venetian and Mediterranean resorts on the eastern shore of Seven Seas Lagoon is roughly the same size as the total Wilderness Lodge footprint (including Villas) just across World Drive, after the constraints of roadway and monorail track are considered. Although this site is a better monorail-resort candidate overall than "Magnolia North" described above, a new monorail stop here would similarly bog down the Resort Loop. Likely road access extending Wilderness Road across World Drive would complicate traffic flow given TTC-bound bus traffic on Topiary Lane. Reportedly, the land's suitability was in question as recently as the 1990s; it's unclear whether such questions have been resolved or how expensive any soil-quality mitigation efforts might be. It's a fair guess that executive whim will determine whether a resort is ever built here. RCID doesn't indicate that the area is a Potential Hotel/Resort space.
8.) WOODPECKER NORTH: This unusually positioned site's acreage approximates the overall footprint of Wilderness Lodge, so a resort would fit, even if the prospect appears counterintuitive. At first glance, the parcel resembles a blister attached to the westbound shoulder of Epcot Center Drive as it bends around the EP parking lot north of Woodpecker Lane. Any access from the principal arterial road would reduce safety and impede the road's level of service, a clear non-option. But close-in zooming on RCID's Future Land Use Map reveals an intriguing element: the possibility of planned access to the parcel from a Hotel/Resort-designated area on the opposite side of Bonnet Creek. Logically, such access would be achieved by a bridge similar to the ones farther south along the waterway. This parcel would be an odd but interesting candidate for resort construction. RCID doesn't show it as a Potential Hotel/Resort space.
9.) Epcot Resorts North: [deleted due to use of parcel for solar-panel Mickey head]
10.) ROYAL OAK SOUTH: This site's acreage is small for a resort--slightly less than the Wilderness Lodge's footprint minus its Villas--but it's larger than the Flamingo Crossings area that has room for three standard hotels, two of which are currently under construction. Extending south from the mobile homes dotted around Royal Oak Court, along the eastern side of Buena Vista Drive and backing to Hyatt's Grand Cypress property in a stair-step shape, the site is protected from roadside view/noise by a lengthy berm. Access is already suggested in the form of Disney Vacation Club Way, extended across Buena Vista Drive as a gravel path. According to Google Earth images there are recent signs of building materials behind the berm, but that could be staging for the new Orange County sheriff's facility under construction across the road. As recently as 1991 this area was earmarked for construction of a support facility--its most likely future use, if any. It's difficult to guess where a detention pond might be located, even as a temporary measure during construction. Chances for a new resort here are remote at best. The area isn't indicated as a Potential Hotel/Resort space.
11.) AK NORTH: This large backstage parcel can be reached only from Western Way, a principal arterial road that lacks the safe access afforded by minor arterial roads, collectors, and other roads (the means by which guests access all current Disney resorts). Further, Disney currently uses the area for a tree farm and other backstage facilities. All told, it's a very unlikely candidate for new resort construction. Even so, RCID indicates that the area is Potential Hotel/Resort/Commercial/Entertainment space; the Entertainment aspect, probably via expansion of AK backstage areas, is the most likely potential use. In their discussion of suitable land RCID mentions an area that could be this parcel, but their meaning is unclear due to a possible directional error.
Finally, there are several Suitable/Mixed Use parcels of land that are of sufficient size but very unlikely to be considered for new resort construction, for various reasons. None of them are shown as Potential Hotel/Resort areas.
Northwest of AKL: This fair-sized, sprawling parcel to the west and north of Kidani Village fits in with Disney's sometime urge to jam facilities into its farthest corners. Depending on interpretation, it's possible that RCID identifies the parcel in their discussion of suitable land. But its current designation as an animal care area fits well with the AKL savanna. Any new hotel construction here would likely be an expansion of the AK Villas.
Southwest of Lake Mabel: This fair-sized parcel is suitable land prominently mentioned in RCID's discussion of same. It's mostly surrounded by private property and Lake Mabel, as well as two interconnecting canals with contiguous power easements, all of which practically strands the area in development limbo. Access between the parcel and central WDW would have to traverse overhead transmission lines, which present at least an aesthetic challenge for Disney; safe clearance under the lines could entail prohibitive constraints for construction crews and equipment. Even if access were achieved, the Fort Wilderness campsites, the Tranquilo Golf Club (Four Seasons), and Golden Oak properties essentially block any potential new roads. A more plausible scenario--though still unlikely--would be for Disney to build an access road on Marginally Suitable land (in conjunction with the wetland-use offset process) along the south bank of canal L-107, westward toward another parcel...
...North of Bay Lake: This parcel surrounds the mobile homes dotted around Bay Court and is accessible from Reams Road. It also stretches toward the Central Shops via a slender connection. Overall, it's of a fair size, but the functional area is reduced by about half due to the mobile homes, whose residents would likely be disturbed by any resort activity. Use of the parcel as a Lake Mabel/Reams Road connector via Bay Court, circumventing the mobile homes, is technically feasible. Even less likely but also technically feasible would be a bridge to the former Persian resort site and thence World Drive. But it's very unlikely that Disney would undertake such complexities to build a resort as far backstage as is the whole area near Reams Road and the Central Shops. The most likely use of this area is for expansion of adjacent facilities.
Car Care Center Environs: This small parcel comprises two almost-connected sections that offer barely enough space to support a smallish resort about the size of BoardWalk Inn. There would be meager aesthetic potential to lodgings overlooking a gas station, a rental car agency, and an electrical substation, even if there were some way to work in a Radiator Springs angle (already done at Art of Animation). The most likely use of this area is for expansion of adjacent facilities or roads.
Northeast of All-Stars: This parcel of barely sufficient size for resort construction lies along the ramp between Osceola Parkway and World Drive (both principal arterial roads), which would preclude easy and safe road access. More important, there's what looks like a pump station near the middle of the area; also, there could be an adjacent underground reservoir, which would preclude surface construction. It's a safe guess that no resort will ever be built here.
All other Suitable/Mixed Use land parcels within the RCID are part of Flamingo Crossings, are adjacent to backstage facilities, or are too small/remote to warrant consideration for a future resort site.
It could be that one day Disney will build on substantial parts of its internal wetlands acreage, counterbalanced by the outlying Disney Preserve and the recent Mira Lago purchase nearby. But logically it's difficult to project that happening in the foreseeable future--more likely it would occur in smaller portions associated with various construction projects, as is currently the case at Disney Springs. For the time being, it looks like Disney will try to maintain 90%-plus capacity in its existing resorts and shepherd Flamingo Crossings hotel construction while it completes Disney Springs, Avatar/Pandora, and the Studios enhancements.
Any differing views? Any additional information? All comments/corrections are welcome!