I'm quite aware that everyone and their kid brother has posted in regards to additional characters. While I hate to join the mob (hypocrite!), I do have two suggestions that I don't believe I have seen anywhere:
Android 16
Dabura
I feel that these two would be excellent additions because a) we need more bad guys (and 16 started off on the Androids' side, stick with me here folks), b) they each have enough attacks to actually make a character that is faithful to the DBZ universe and not a thought exercise (what if Raditz had gone SSJ, etc.), and c) you can feasibly get a transformation for each without bending logic (Mystic Krillin is cool, but why aren't the Z fighters upset about him aging so quick?).
Finally, I just think that both of these guys would look very cool in game, and relish the thought of giving Cell a Hells Flash or Buu a good old spear through the stomach.
What do the rest of you think?