After Everest opens, I would still love to see more of an expansion at AK, especially if we're talking about a completely new land, whether it's Australia, or whatever, but MGM probably makes the most sense for the newest big attraction.
With the new stunt show set to open, Indiana Jones Stunt Show becomes expendable, since it's been in the park since the beginning, and I don't see any need for 2 stunt shows in the same park. With the new Indiana Jones movie supposedly coming out, it would seem logical to get rid of the show, and add a new version of the IJA from Disneyland.
Also, I personally LOVE the idea of the Muppets Great Movie Ride, and I would love to see the whole Muppets land carried out, with the restaurant that was described and everything. With Disney owning the rights to the Muppets now... hopefully that, with a strong showing of the Muppets Wizard of Oz will make this possible. But, we'll just have to wait and see.
Regardless, the next big attraction will have to be somewhere towards that end of the park, since there's a great imbalance in the crowds. You have 75% of the people on Sunset Bloulevard for the entire day, including when they all pile in for Fantasmic. New attractions at the opposite end of the park will spread out the crowds a little more, and with more stuff to do, it would be possible to go to MGM and stay for more than half the day without getting bored!