I think this sounds like fun, though the theme should be optional (if it isn't already). As said already, not everyone can afford to make lots of costumes, or have the time, even if the outfits are fairly simple.
I don't think I'd have time to make a new outfit every month anyway. I'm currently working on some outfits right now (a B-Wing pilot from "Star Wars", an Earth Federation uniform from "Mobile Suit Gundam" and a Mayuri Kurotsuchi from "Bleach").
But a monthly, bi-monthly, ect. meet-up would be fun.