Spell Casting might be a little out there, since the style of the game isn't so much magic, but instead technology.
Alternatively (And this is probably the better suggestion) Bombs are sort of the "Caster" weapon. I think the addition of a bomb that heals players could be really nice for a Cleric type class.
A healing-bomb would break the balance of the game. Also, I love the fact that you can't heal others. Everyone being a damage dealer (or sometimes primarily support with particular bombs) is a really awesome thing. No one gets left out and is right in there with the action.