Call me crazy, but I've always thought Seaworld could benefit from a small indoor family coaster. There's a ride at Kennywood (big fan of that park myself, Izzy) called The Exterminator, which is an off-the-shelf Reverchon spinning mouse that's been put inside of a building, themed decently, and much of it is in the dark. It's SO much fun. I've always thought that if Seaworld went with something a bit more unique, such as an Intamin spinning mouse (INSANELY fun and only one has been built at Bakken, called "Tornado"
http://rcdb.com/4313.htm) and themed it well inside of a building, it could be a wonderful addition to the park. I also wouldn't mind seeing them build a B&M wing coaster, a decent water ride, or a large, thrilling flat ride. Themed well, it's amazing what you can build in terms of a mega flat ride. There are some great examples on Intamin's website, but it seems to be down at the moment.