That's pretty much what they're doing. Batman, Superman and Aquaman all have the same special moves, stances and combo strings from the first game.
the problem is that there's too many "must have" characters in a DCU game.
I really do have to wonder if the majority that complain about some rosters being too small in fighting games even bother using at least half the characters even if the roster was bigger. I remember a couple years ago I was playing Ultra Street Fighter 4 online and a good 75% of the matches was me fighting Ryu, Ken, Akuma or just about any of the other shotos. The 44 character roster there may as well been a 7-8 roster. People complain about how small a roster is, but then seem to end up using the same character 24/7 regardless of how big it is.
one of the reasons for that is how theyre involved in story mode (in mk). what if the mkx roster was just scorpion, sub-zero, and like 5 more characters? i dont think you would like that very much.