Well in short, Rapture is simply the "project" name for the new MMO SE has been working on recently. It's going to be host to their new "Crystal Tools" engine and SE was nice enough to release a few photos of it's capacity.
Unfortunately other than that there's not much we can say about it. SE is very good at keeping things in the lowdown.
PS: I really like your ideas Pale. Sounds like something Bethesda would do, and it's definately something SE should contemplate.