In addition to the interior, it would need an excellent exterior. Lots of lights are a must. ...
Here, here! Back in the day I spent a fair bit of time on both sides of the curtain (and in the lighting net, the control booth, under the stage, in the limelight, in the carpentry shop, in the costume shop, etc.) and I'd love a theatre that actually respects the complex character of a real opera house rather than a simple auditorium with a raised platform in front. My dream theatre set would have: a costume, make-up and property rooms; a grand stage with stage elevator and trap doors, a working curtain, raisable backdrops, a lighting array and pass-thru; an auditorium with orchestra and balcony seating, and an orchestra pit; a control room/projection booth; and, of course, a stately lobby, lounge and classic exterior.
Size (and therefore price), of course would be an issue, I could see a 'good' theatre starting at two full baseplates and being twice the size of this year's Town Hall. It wouldn't have the piece count of a Taj Mahal (simply because so much of it would be empty inside) but it would need a much bigger footprint than the typical modular to even play at being a true theatre.
A cinema would be easier (having worked there as well) as it doesn't have nearly as much infrastructure going on and could certainly be made to fit in the typical Modular footprint. There's an old Art Deco movie house near me that might make a good model.