Another possibility is that there is an ancient evil that Raven has been keeping at bay. But by raising Harry, this evil senses an opportunity. This antagonist could be something that was once sealed away long ago and now wants to use Harry's power (which has been shaped by Raven) for its own dark purposes. It could be an entity that is more powerful and mysterious than Voldemort, leading to a very different kind of struggle for Harry.
It could be the same Voldemort as in the original series. But his approach to Harry might be different. Since Harry is being raised by Raven, he might be more difficult to target as he has different knowledge and skills. So Voldemort might have to come up with new and more devious plans to get to Harry.