For the 50th, MK is getting:
- TRON Cycle,
- TRON Racers,
- the completion of the Tomorrowland update
- cleaned up public spaces,
- Stitch replacement,
- refreshes to CoP and TTA
- new night time parade
- new (or HEA-updated) fireworks
The new theater has been axed, but there are hints of one other major attraction update or addition (but considering the theater's axed... don't hold your breath).
Also, the 50th for the MK is also being considered a 50th for all of WDW, and so, the GotG ride will be considered part of it. The 50th celebration will bleed into 2022 and then incorporate Epcot's 40th. Hopefully, the spine re-do will be done as well as Brazil and maybe whatever seems to be ruminating with WoL pavilion.
And don't think that WDW won't market all the stuff coming online in the past few years leading up to the 50th won't get counted as part of the 50th celebration.