Its still up in the air at the moment. There are two big forces pushing it.
One is that Disney is building an impressive list of stage shows between
Disneyland, the cruise line and Broadway. Rolling in a new stage show has always been a very cost effective way adding new attractions to the parks. Already California Adventure has seen several large production shows come and go (but Disneylands first attempt Snow White was a major flop). Having a more general purpose theater would help the Studios keep adding new.
The second is revenue growth. Never underestimate the corporate pressure on WDW to continue churning out the cash. Its warping the very nature of WDW. Disneys hit a limit on admission, dining and lodging prices so they have to come up with new ways of hitting you up for money. Its the cruise ship mentality, buying a ticket is just your opportunity to spend even more money. So Disney is hitting up the photographs, charging for formerly free services, staging after park hours parties with a separate ticket all sorts of the revenue enhancements.
Disney knows it can pull in a hundred bucks a ticket with a Broadway show, so it makes such to them to begin upcharging where they can. A theater can house a theme park show during the day, be turned into a full scale theater at night. That was the original concept behind the Hyperion Theater at California Adventure, but budget cuts so gutted the theater its not even suitable as park theater anymore.
One can assume that at $150 for the Broadway version of
Tarzan, Disney would be willing to build a real theater at Disney/MGM Studios.