When Avatar Land opens, and then Star Wars Land follows, everyone will be happy...well, for a day. Then they will immediately demand more.
There's some truth to that, but it isn't because the people on this board are malcontents, as your post would imply.
Most local theme parks around the country get a new attraction every year and most get a major addition every five years. The WDW parks are woefully behind on that schedule and TDO has believed for too many years in the "one resort" philosophy (meaning one new attraction feeds all 4 parks, not just the one). This philosophy has made the parks incredibly stale to the point that even a major addition or two isn't enough to compensate for 20+ years of neglect.
Here's a list of what, IMO, is needed for WDW to be up-to-date (and what it would take to get the fanbase truly happy):
• Hollywood Studios is in desperate need of more attractions. They currently have a "land" dedicated to Pixar with one attraction (that usually has a 60+ minute wait). A Monsters Inc ride, an upgrade of HISTK playground to A Bugs Life and (would be nice, but not holding my breath) an Incredibles ride would go a long way to making that park (along with Star Wars) finally a full day park. Star Wars would be amazing, but more is needed to truly round-out that park.
• EPCOT: FutureWorld is plagued by outdated and closed attractions. The list is too long for needs for FutureWorld so I won't do a full list. An upgraded Imagination, EO replacement, and Ellen's Energy replacement would be the first places to start (and would probably placate most hard core fans for awhile).
• EPCOT: World Showcase hasn't had a new country added in 25 years. It's time (especially since many of the countries lack rides or major attractions). Adding a new country (or two) is desperately needed. Upgrading many of the key attractions in the existing countries is desperately needed as well, since many haven't been updated in years.
• MK: with the addition of the FLE, MK is the one of the least of the parks needing anything but Tomorrowland has grown incredibly stale and is showing it's age. A new attraction in TL (bye-bye Stitch please!) would go a long way. A new glass roof for Teacups would be nice, too!
• AK: Avatar and the new nighttime show should give this park what it needs for a while.