3) Reverse-Flash
Ok, so he’s not a hero, but with villains like Captain Cold and Heat Wave set to play a key role in Legends of Tomorrow, why can’t Eobard Thawne show up? After all, the season one finale of The Flash made it clear that he has history with both Rip Hunter and the Earth-2 Flash, so him being plucked out of time to help out the team at some point could work brilliantly, especially if it somehow leads to him being able to make a return in The Flash down the line.
Regardless of whether this is the Wells or Thawne version of Reverse-Flash (the latter would be fun), watching him interact with the team – particularly Dr. Martin Stein – would make the episode he appears in a must-watch.