I know I've said it before, but I'd love to see one that would spawn off of any of the diagrams in the forums to the position currently being displayed (so if you're working through a game or puzzle you can start the analysis right from the position you're seeing).
Incidentally, if this were impemeted you could fulfil your request by simply creating a private blog post with a board in the initial position and spawn it from there, but it would be even better if any of these boards from the forums (or from your games) had the option to be reset to the original position and to have pieces added/removed at will.
I can't tell you how many times a day I would love to be able to pop up an "analysis board" pane to work out moves for some position I'm looking at in the forums. How hard would it be to have something accessible from the main navigation bar (play/learn/read/forums/resources/etc.... ) that was just a board at the starting position where you could either play from the starting position, or set up a position, (or load a fen/pgn?) and analyze....
there's a 'work-around' way to do this now (that I use, btw) which is to reply to a post and insert a game into my reply, and then work with the game insertation/annotation interface as an analysis board, but it's a dopey, clumsy way to go. And I'd think giving us a convenient free analysis board to use would actually be pretty easy to do.
Think about it.
And folks, if you read this and think "hey, that's a good idea!" -- say something and put a response in here, cuz the powers-that-be definitely respond to the will of the masses.