Going by what I could see, the stained glass structure's usable interior space seems far too small for a show of any decent size. Below is an image comparison between the inside of it and the inside of the Dragons theater. Neither includes their support buildings beside them though.
I still think there's space for a small queue in the adjoining structure, and only enough space for something like a carousel (or a similarly sized flat ride) inside the main structure. The interior usable space is just about equal to the smallest quick service in the hub, which has around a dozen tables and serves ice cream. If this mystery building is in fact for a show, it will probably be pretty low capacity.
There are a couple larger structures near the park's entrance that might be able to contain a show, one of which does not appear in the concept art. (The ones that are in the concept art likely aren't tall enough for such a thing.) And that other building appears to be OUTSIDE of the park gate, so, certainly could be a separate ticketed thing, but shares the parking lot... (Or, it's just a park support building, I have no idea, most of what I've seen only show foundations and not heights so there's no telling.)
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