First my disclaimer:
These are rumors and I do not know if/when these will come true.
I have a good neighbor who's Uncle has worked at the MK for nearly 15 years (Splash Mountain facilities). She just came back from visiting the MK and he swears:
Space Mountain will close for nearly a year for a significant renovation in 2009 (I actually think I have heared this other places).
The Riverboat will be removed for good in the not too distant future (not happy about this but I do understand that it seems to take a great deal of upkeep).
The Fort Langhorn and other feature on Tom Sawyer Island will be removed similarly to what has happened at Disneyland (not happy about this at all-
).
Big Thunder Mountain will be basically leveled and re-built on the same spot as part of a huge renovation at some point in the next few years (that is a ton of $$
).
If I hear anything else, I'll post.