3) Max Mercury
Barry Allen may win the award for Most Father Figures In A Single Television Show, what with Henry Allen, Joe West and Harrison Wells, but – hey – he could always use another, right?
In the comics, Max Mercury is one of DC’s oldest speedsters – he was originally called Quicksilver when he was introduced in 1940, but Marvel later nabbed the name. As a veteran hero, he’s often a mentor to the Flash in the comics. He’s also an expert on the Speed Force, which was finally introduced properly at the end of season two. Given this, it would seem that season three would be a perfect time to bring Max into the show.
Alternatively, his body was stolen by evil speedster The Rival in the comics, so he could be used as next season’s big bad instead. If nothing else, the character would definitely lend himself to the “trusted friend is actually the villain” template that The Flash‘s writers clearly love.