If Leto bombs and people dislike his performance like Luthor in BVS, I could see WB turning him into Jason Todd and getting a new actor to play the "real" Joker. Unlikely, but not impossible.
It wouldn't be the first time a studio makes a change because of the fan backlash, look at the Mandarin change after "fans" hated the fake Mandarin twist un Iron Man 3 or how Shreder was changed from a white actor to an unknown Asian actor in the 2014 TMNT movie because of the backlash...and in the sequel he's played by yet another actor.
I doubt Leto shaved his eyebrows, gained weight, spent hours in the make-up chair and said no to Dr. Strange (supposedly) to play a Joker fanboy in a 5 minute cameo and let some other actor play DC's most popular villain, unless they have long term plans in mind and several movie appearances planned....that's of course if the audience likes him, if not WB will get rid of hin.