I like MGM's idea about the 2 player skins. That's pretty cool. Another character I would like to see...Evil-Lyn. From the sounds of things, she isn't one of the villains you do battle with, and she betrays Skeletor all the time, so playing her in the game would be like the MYP episode when she's trying to escape Skeletor and his Evil Warriors. That would be pretty cool.
Other than She-Ra, Teela and Evil-Lyn...I don't know who else I would really want as a playable character. King Randor or Fisto might be cool (and both have swords, so it should be easier to insert them into the game). It would be fun to punch people with that giant, metal fist!