Beyond casting The Flash himself, his 'Rogues Gallery' and extended 'Flash Family' is larger than almost any other DC character; as such, the amount of casting depends largely on the studio's long-term plans for the character. It's likely the first film will be limited to Barry and his love interest Iris West to start with, but even so, a proper villain will be needed.
The most obvious answer is The Flash's arch-nemesis (and aptly-named) Reverse-Flash, but a villain who plays such a pivotal role in the larger mythology may be saved for later. That still leaves the likes of Captain Cold, Captain Boomerang, Heat Wave, Trickster, Pied Piper and others - all offbeat (and deadly) antagonists that could make up an imposing ensemble cast.
An odd mix of humor and depravity defines the Rogues Gallery, meaning more glimpses into the twisted mind of the super-criminal community.