I want that to happen. It would be wonderful. I don't want a love triangle though.
Squeenix is being tight lipped about this because there has to be some major plot point they don't want leaked re: these two. It's quite possible that They are twin sisters & rivals. It's very possible one impersonates the other for devious reasons.
Pumpkin's split personality suggestion would be an interesting twist.
Interesting though it may be, I somehow doubt it. Considering it's a fantasy based on reality, I think they'll avoid campy anime tropes. Unless they really delve into the psychology of it a la Black Swan or something. That would be awesome.
And I repeat for all those saying that her not being in the demo means something - everyone at SE saying "I can't comment on that" means something is definitely going on with Steplla's character that they can't disclose. Also, I don't see anyone doubting Cor's existence