(Reposted from VWTheory's themepark section.)
I was thinking about this tonight, why do we create systems of "levels" in MMOs, then restrict those levels to "zones" in the meager hope that people will play through our content in the correct order? Why not just make Levels in the traditional sense, and as players progress through levels they get additional gear(think Zelda's Boomerang, not Shaman's boots) and so forth. Players then have to complete a level to move on, but would be rewarded for coming back to old levels with stuff that might be either universally useful, or useful on whatever level they were currently on.
Granted there are problems with replayability, but since we are already talking about themeparks, why not focus on making better rides? You would need to focus on your social elements, avatar customization, houses and leaderboards for example, but if you can get them to feel some meaning in the their persistence it could very well take off.