Reasons for further expanding Magic Kingdom:
(1) People view MK as the "main" park, representative of Disney, or even just "Disney" (with the other parks being "owned by Disney" or whatnot). As such, guests' experience at MK often reflect on their perceptions of Disney much more than anything else, and thus MK seems key to maintaining a positive public perception of WDW as "must do" (especially with increased competition, etc). New land(s) and new e-ticket(s) would serve this end.
(2) MK is crowded, and new/expanded lands, as well as new attractions, will help spread the crowds (rather than only having major new things in the other parks, which inevitably draw people back into MK as well, since it's the "main"/"must-see" park, etc.
I would vote for IJA in an expanded Adventureland (south of Splash), and JttCotE in a Mysterious Island or other new land (perhaps with an entrance where Autopia is now).
A controversial thought: Are everyday guests' experiences lessened by an over abundance of c-tickets and below at MK (when they don't know which are the major attractions, and thus select things randomly, thus causing their visits to be filled with 2nd-tier attractions)? If so, should new e-tickets purposely replace lower-quality/lower-tier attractions, rather than just being added elsewhere? (This is of course at odds with capacity concerns, but perhaps a balance can be found...)