Yeah but here's a couple things that you have to consider...
Ed Boon said that they considered putting Professor Zoom in the game but he's pretty much just The Flash in a yellow costume so people like the other Green Lanterns, batgirls and robins won't be put into the game because they're just the same... Honestly, u don't even think robin will be in the game because nightwing already covers that with his staff stance.
They aren't all the same though. Each Robin is different, and could offer different specials, different personality when it comes to how they act and role they play in story, and each looks different. It's tough because i'm not sure of everyone's individual knowledge of the characters. To best lay it out though, to show my point:
Nightwing (Dick Grayson, the first Robin):
He uses his Escrima sticks in unique fashion. His personality is very strong willed and Independent, but extremely loyal to Bruce. He's usually pretty y and brash, with something sarcastic to say to play off Bruce's more serious approach.
Red Hood (Jason Todd, the 2nd Robin):
He was troubled youth, which led to him being a bit more stubborn. Of course, he died by the Joker's hands before Batman could save him, which led to him becoming revived by the Lazarus Pits, and taking the role as the Red Hood. He is an anti-hero, and is similar to Batman, but he isn't affraid to kill if necessary. He is usually very serious in tone, and his weapons include guns, knife, etc.
Red Robin (Tim Drake, the 3rd Robin)
Tim is a member of the Teen Titans. He takes the role of Red Robin, with his main weapon being the staff. He's seen as the smartest Robin in terms of intellect, and shres similar qualities as other Bat family members: "The intellect and detective skills of Bruce, the ability to lead others and be a friend to others like Dick has and even the ability to make cold, harsh decisions like Jason does." Making him extremely well rounded.
Robin (Damian Wayne, Bruce's & Talia's son, 5th and current Robin)
He was trained by the League of Assassins at a young age, and thus, he kills. Which causes a troubled relationship with Bruce, but he still loves him. He is trained in every weapon known to man, and uses a katana as his main weapon of choice. He is an expert in impersonating voices and engineering. Able to get into the Bat cave.
So yeah...they may all be Robin at some point in the comics, but they are completely different from each other. Personality, skill sets, purpose, morals, choice in weapon, and appearance etc.
The Green Lanterns and other people who take on similar roles over the years, have similar diversity. Just because they take the role of a hero, doesn't mean they all act the same, use the same methods, have the same morals and values, look the same etc.
Not saying we need a roster of 4 Green Lanterns etc. But to discard others who are different and could serve different roles in the story and play differently, just because they have shared the same role at one point, doesn't seem like a good reason.
This thread is about character possibilities, and that's why I listed them. They would make great characters.