The theatre in TWODW is there to be used for entertainment once again rather than some 4D show. I have no doubt that it will be put to use in the future or at least see something happen with it once Project Horizons happens though even if they did, it would likely be just for families and the adults won't have their fill.
I agree that the park will have to appeal to a more mature crowd either at the hotels or maybe during the height of summer an area of the park could have a theatre built, or rather some of the many empty spaces on the park which we can have our pick of, can have a show aimed at an older audience that can be performed during the busy summer season.