A little while ago at the Saturn Awards, Grant Gustin, who plays Barry Allen on CW's The Flash was asked about what was to come in season 2 of the show which is due out this October. Gustin said:
Yeah, we're introducing a really cool big bad that's gonna be revealed kind of immediately... So that will be really cool.
While it is unknown who the big bad will be at this time I have heard that they will be crossing over with Legends of Tomorrow which as of right now only leaves one option: Vandal Savage.
While no word on this has been officially confirmed, I still think that it would be cool for Savage to be the main villain on both shows.