This is interesting. The is the proposed conditions map that was used to design the new storm water control improvements. The "D" color is designated as "impervious" land. Water is marked in blue, and it's interesting to note that the entire river is gone except the one small part. I thought this might be because the river has a concrete bottom, but the second picture shows it as water not impervious.
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I'm not familiar with Florida's stormwater code specifically, but this appears to be more of a future-proofing exercise than reflective of any meaningful plans.
Typically, stormwater management is required when the ground cover changes from pervious (forested, landscaped, grass, etc.) to impervious (buildings, roads, walkways, treated pools/water features, etc.), in order to capture the additional runoff from rainfall that is no longer absorbed by the soil. This is done to protect the downstream channels from erosion with high-velocity flows, and from flooding with increased runoff volume. Essentially the goal is to make sure the post-developed runoff doesn't increase and cause problems on someone else's property or for the natural systems.
The exact type of stormwater management facility can vary from surface ponds and canals to restored wetlands to underground vaults, but they all function in a similar manner: as a holding facility to detain the additional runoff and release it slowly over an extended period of time, reducing the peak flow rate. Similarly, these can be small facilities designed to capture a single project (say, a new restaurant) or large regional facilities to include future development with known or unknown designs (like and office park or residential subdivision that will get built-out over several years).
Given that the existing ROA is part of the existing stormwater management network, removing and replacing its capacity will be a large undertaking. A smaller-scale but similar project was undertaken ahead of the Tron construction to modify the ponds on the east side of the park, and even that was expensive and time consuming. While earthmoving is a costly operation, the incremental cost of increasing the size of a facility you're building anyway is relatively small, since so much of the cost is associated with design & permitting, getting crews out, and simply moving the dirt around; a facility that's 20% larger may only cost 5% more.
With that in mind, it makes sense that they want to use a regional facility that assumes the entire park is impervious. This will be a once-in-a-generation stormwater project anyway, so why not make sure that all the bases are covered for the foreseeable future. Realistically, it removes the stormwater management burden from small projects like walkway widening, restaurant expansions, and (yes) new DVC kiosks, which is probably the primary goal of the project. But it would also allow larger changes, like finally putting that climate-controlled dome over the entire park.
I wouldn't read too much into the image showing the entire park as impervious surface as meaning they have concrete plans to do that. It's just allowing them the flexibility to do whatever they want in that area in the future (at least, until a major revision to the stormwater code), without having to modify the stormwater system each and every time.
