I'm not sure it would actually be a good idea. As much as I love Faith (and trust me I do, A LOT :silly🙂 I think the whole Faith spin off opportunity is gone by now. I mean, we have BtVS season 8 that already includes Faith (as far as I've heard cos I didn't get to that part yet) and I think, like JCC said, that both Joss and Eliza probably want a new challenge, a new story, and as great as Faith was, they have to move on because things that work in an original production (of a show or a movie) doesn't usually works as well in a sequel. Fair enough, Angel worked out fine, but that was because the whole story arc was calculated and everything fitted together you know?
The only thing I can think of is invastigating Faith's past and that obviously would not be with Eliza (or probably not) so I don't think it would be worth it :silly:
But then again, you never know. I'm just really content with the Dollhouse idea so far. I'm sure Joss will manage to create a new world with the same success... as long as he doesn't lose his humour and critical ideas... and his genious of course :silly: