Here's why shapeshifters will never be in fighting games, whether it be Beast Boy, Martian Manhunter, Animal Man, etc:
Think of all the possible animal forms you could add for Beast Boy. Now lets say a player were to do a combo using some of the various animal forms. Here's what I mean by that
A = Lion
B = Tiger
C = Bear
Possible combos
A to B
or
A to C
or
B to C
etc,
You get the point. That was with just three forms, there would probably be more, so consider that as well as every time he isn't doing a move, he's in human form. Or if he gets hit in one his animal forms. The point I'm trying to get across is it's REALLY impractical to make such a concept work. Think about all the animal types and combos. Every time he changes from one form to any other form, there has to be a really complex animation. It would take a REALLY long time to make that function probably. Also if some of you are thinking that they could just have a flash or something, it would only work on the surface, the bones in the animation would still need to be changed as if the flash wasn't there.
Lets say for the sake of argument you have the time, resources and energy to do this (for just one character BTW. Not exactly a great payoff in the grand scheme of development) you have to make it function with the game's mechanics. These fights are relatively fast-paced, it would look really odd for Beast Boy to change forms constantly like that. Also what if BB gets hit in between changes, you'll need another animation or something to fix that. Or what is Batman or someone does their super move on him in Lion form, what then? The victim part of the super move is somewhat generic, so having a lion or something like that in there would throw the whole system off.
Do you guys understand what I mean? It's just too much work for one character slot! It's just not worth it in my opinion.
