Rebecca Hall already played a character in IM 3, so she's probably off the table at this point. I also subscribe to the theory that Skye might turn out to be Jessica. It would fit and explain some things about her character. HYDRA being so heavily involved only makes me think it even more. Plus, she's an obscure enough character to the non-comic book reading public that they could probably get away with having her in a TV show, or at least introducing her there. Also, didn't someone at Marvel recently heavily imply that Jessica might be getting her own book later on this year? I'm pretty sure that someone did.