Leaked gameplay of Injustice 2 shows off the Joker. Not much is known at the moment except for he was going to be announced on April 28th. But looks like we get a small look into his character before the big revel on the 28th.
Although since the achievement list for this game was announced and this achievement: "Why Aren't You Laughing?" and the description being: Joker must defeat Batman with the final hit coming from Not So Funny Is It? It's not really a big leak.