It would be really interesting. Her main powers is to copy others and take their powers, right? So if we had Mystique as a starter hero, wouldn't it solve every problem ever? Instead of having a skill tree of her own, she could literally copy another character. Aim at Captain America, and boom! Now you're playing Captain America, complete with his skill trees, his costume (at least the one the other player is using), his gear, his everything. Want to try Iron Man? Change into him, and there you go. Now you're Iron Man, complete with his voice, skills, gear, you name it.
What this would do is it would allow a new player to try out every hero in the game for free - but only one at a time. Sure, you could turn into Angela on release date and never change into another hero if you don't want to. You would even get her gear, her alternate costume (if the other player is using it) and so on. But once you change into another hero, you've lost all your Angela-ism, meaning you can't wear her gear, can't use her skills and all that. If you want to change back into her, you would need to find someone who's playing her, and you would loose the current apperance.
Plus, on top of this, it would solve the big issue how to get Mystique playable in the first place. I like Rogue a lot in this game, but I'm not sure about only borrowing or stealing one power from each hero. I think she could rather steal a whole bunch of powers from each. Maybe even have five or six powers from each, and we get a power at random. But if Mystique was a starter hero, she could literally turn into anyone. Sure she won't have a skill tree, gear and all that, but does she really need it? She would be really true to the comic book version, help new players AND be really unique. Win-win-win. :)