Note: This post is mostly speculation. It educated guesswork, and I could easily be wrong on some or all of the points I will make here.
I believe that Hollyleaf will turn up again in one of the books in Omen of the Stars, probably at the end of the book. (For a suspensful ending) It does seem possible that she could become one of the main antagonists in the upcomming series, although she is likely in a very fragile state right now and could go either way. Perhaps she will meet up with Sol. She might feel as though she was betrayed by the Warrior code, and could be easily influenced by him in her weakened state. Or, on the other hand, she could still be obsessed with the warrior code and want to get revenge on those who have broken it. Perhaps she will want to take over all the clans and strictly enforce the warrior code. (Although that's pretty much just a wild guess)
There is also the possibility that she will not "Turn Evil" but instead go out to live as a rouge. She is not likely to return to Thunderclan however, due to the fact that it was Thunderclan that she was running away from when she went into the tunnels in the first place. Whatever the case, Hollyleaf will probably have an important role in the comming series. After all, if she didn't have an important role why have her survive the collapse?