I think Rebel Jill is possible when the She and Tremolo starting to explore the secret about her sister Lana. I mean, all this "Alternate Jill" talk, in a nutshell, it's just her sister. Idk, maybe Lana secretly alive or something. Or she's a former girlfriend of Bella, and she died, resulting with Bella that become so strict (we know how laidback she was in the past) and how Bella and Jill become close (maybe Bella promised to protect her gf lil' sister or something). Like you said, for me, the big room for Jill to fill up, at least for me is one of many things of her that i find really interesting.
Then again, in terms of gameplay, i still consider her as the worst (if you full go with the CHICK route), because you're going to miss SO MANY things in the game just for her. She's still the best girl, tho. Just, i hope DPC can toughen her up a 'lil bit.