About a week or 2 ago I was on Injustice's Facebook or Twitter Page and someone asked them if the Next Character will be in the 1st Season Pass, and they said no. I didn't snap a pic, but I saw it. That doesn't mean they didn't change there mind though.
P.S: The release date for MM and a 2nd Season Pass is still unknown, but my guess is that Yes, there will be another Season Pass and MM will come out next Tuesday.