The great thing about Fables is that whenever they add a new character into it, you're immediately already interested because you're familiar with them. They don't have to be a long standing character to start out.
I'm guessing they'll have a new Fable from some fairy-tale not yet in the comics as the protagonist. This would allow them to "own" a character, do whatever they like with him, and not interfere with the books' story. This could be anyone of course, but for the fun of speculating (and yes, I know a few of these are problematic with previous cameo appearances, but that can be worked around):
Wicked witch of the West
Bone (still hoping they'll continue this series :-P )