MK- I have to agree with the Bald Mountain thing. I have wanted this for a while. Not really a flume though. Some water, but mostly like a roller coaster
Epcot- Any of the rumors floating around really. I am especially fond of the StormRider though...
MGM- A coaster in the back of the park, possibly themed to Monsters Inc. or maybe that Villians ride idea, only added to instead of taking out tGMR.
AK- Don't get me started on this. I, for one, would love to see a Tarzan themed roller coaster, esecially a stand up where you are doing the tree-surfing thing... or a good flume ride in the Africa section due to the fact that AK is the hottest of the four parks because of the foliage.