A fire elemental shouldn't be immune to fire, it should eat that Fireball you just tossed at it and grow stronger.
Thematically, perhaps. But, as a game element, it is kind of pointless. That old school, "Surprise, you just healed the monster!" only works once, and the gotcha isn't all that impressive.
And, I don't want PCs bringing up elementals, and then healing those elementals. If I want a baddie able to heal elementals, I'll give them a power/spell that does that specifically.
