It appears as if Injustice is getting more DLC in the form of new S.T.A.R. Labs missions, which are basically akin to a challenge mode. Specifically, on July 23, each new DLC character (Lobo, Batgirl, Scorpion and Zod) will get 10 new missions each to mess around with. If you're a Season Pass owner, the content is free, but if you're not, this costs $1.99.
I'm a big fan of challenges, since they're a great way to break up the action every 100-200 matches or so, and they can teach you new tactics you never would have tested otherwise.