Would it be plausible for Microsoft to requisition a new franchise, and allow Insomniac to keep the rights to it? Now that I think about it, that question is dumb. If MS is simply publishing it ala Mass Effect. I also can't see Microsoft creating a new franchise, only to let it walk away.
I think this is what is happening, actually. Like a new Gears of War. Insomniac owns the IP, but Microsoft will have first right of refusal for sequels. I think a partnership with Microsoft makes a lot of sense for Insomniac. Their games will get more attention than they ever have. Microsoft can do a better job promoting and marketing the game in the media than Sony ever could, especially during the early gen.
And I don't think they're working on a platformer or cutesy shooter or whatever, but we'll see.