Greener grass & fences!
I would suggest that to make a better RPG the players should be normal beings in whatever setting you choose, NOT planeswalkers.
While that's not the kind of game I want to run overall, I have to say that it is a really cool idea! I might use it for a sub-plot.In fact, an awesome transition of power could occur one day when the players are all summoned into a battle by a planeswalker, declare "I've had enough of this kind of slavery of my people" and turn around and lop off the head of the sap who summoned them, thus declaring a revolution of sorts between the planeswalkers and their poor victims.