You're thinking of building one?
Theatres are expensive to build and to run.
Some sorts are much more expensive than others to build; some sorts much more expensive to run.
An institution thinking of building one, or incorporating a space suitable for live theatre in a new building, must have a fair amount of capital in hand, an assured cash-flow to buffer the erratic nature of income from the box-office (or from theatre hire, if that's an option) and a certainty that it will be used regularly. Some institutions have a more or less captive and well-trained audience or clientele. Others have to work at it.
Unless the institution is a drama school, the design of the new theatre should provide for alternative uses and thus alternative sources of income when it would otherwise be 'dark'. The essential form of the theatre depends absolutely on the functions it will be called on to fill.
What other uses are guaranteed, likely, possible, unlikely, impossible? If it is to cater adequately for its intended pattern of use, how big should it be, ideally speaking? How big could it be, practically speaking? Is there room on the site? Can it be sited so that one doesn't need a pocket satellite navigation system and an armed escort to find it in the dark?
Other factors to be considered include such mundane matters as accessibility by public and private transport, and how to build a community perception that this will be a good place to go. The theatre's front-of-house philosophy has to start with an understanding that it's comfortable at home on the sofa in front of the wide-screen digital TV with surround sound and a glass in the hand.
What theatre form would suit you best?
Theatres come in an infinite variety of forms, but they can be clumped together into about nine categories: proscenium-stage, open end-stage, corner-stage, thrust-stage, kabuki- or caliper-stage, centre-stage, arena-stage, black-box and variable.
As a generality, one can say that each of these forms is excellent for some sorts of event, but impossible for others.
(and so on...)