Peek-A-Boo was not always a criminal. The teleporting Lashawn Baez was caught by The Flash actually resulted in creating a villain. She was trying to steal from Central City Hospital to help her ailing father get a replacement kidney. It would be amazing to see a female villain with the same level of super power go one on one with the Scarlet Speedster.
The Green Lantern has already been introduced to movie audiences but a reboot Green Lantern or just a cameo of the hero in The Flash film would be a great little feature. As Green Lantern, Green Arrow and The Flash are friends in the comics.
Chester P.Runk, a brilliant cosmologist got into an accident with a matter transmitting machine left him like a human black hole. He used this power on anyone who he did not like which included The Flash. He, later on, turned sides to become one of the good guys and become friends with Barry Allen as well. This complicated relationship would be a great focal point for the movie.