The deadline for the biennial Soho Theatre Verity Bargate Award will probably be in August. Rather than wait until the the award is officially announced, I'm outlining my ideas now and researching them. I'm dying to tell you what they are but I won't. It's not that I don't trust you - I do, with my life - it's just that talking to much about an idea can kill it a bit and make it harder to write. Especially when your so called partner and friends are non-plussed about it: "Is that it?"
Luckily, theatre is probably the most free of the scriptwriting arts and so don't feel too restricted regarding subject matter or genre or even structure. It can be something personal to you and what you want to write about and how you want to write it.
While you don't have to take into account commercial breaks and censorship, and you can play with structure and non-naturalism, I wouldn't recommend throwing out the dramaturgy rule book entirely. Give your imagination full-reign by all means but I find it works best with some structure - however basic - and good characters.
If you want to write for the stage but feel you can't because you don't watch much theatre then I'm not accepting that as an excuse. Sorry. Firstly, see what amateur shows are on, they'd be cheap and a good introduction. Secondly, I'd recommend reading some playscripts at your friendly neighbourhood library. I believe the Verity Bargate winners have been published. If you're used to writing for a camera with lots of sets and lots of characters then perhaps the stage seems restrictive but reading those scripts will show you just how much you can do and also show how every writer is different and you can just be yourself.
Personally I can't get over the habit of minimising the sets and minimising the cast list in my plays as I feel it's more likely to be produced. However, the theatre bosses are crying out for big new plays by new writers which they can run alongside Shakespeare on the big stage and not be put away in the smaller studio. The choice is yours. Or rather the choice is within the idea you have as I think each idea suggests the genre and the best way to approach it.
The Playwriting Seminars
One of the classic web resources - tells you everything you need to know