6) Eddie Thawne
When we last saw Eddie Thawne, he shot himself through the heart in order to wipe his descendent Eobard (a.k.a. Reverse-Flash) from history. His body was then sucked into the Singularity, and who knows what sort of changes it could have gone while passing through that doorway.
In fact, there’s a chance that Earth-2 technology could have been used to save his life, and if that’s the case, it means that the Reverse-Flash could make a return somewhere down the line, too.
However, with Iris and Barry seemingly about to finally embark on a romantic relationship, Eddie’s return would throw a real spanner into the works for them, and if he has been given powers of some sort, his presence in season three of The Flash could shake things up in a very big way.