On dealing with element immune monsters, being useless in an encounter isn't fun, but having your role change in a certain type of encounter can be. What if Kineticists get an infusion that lets them level/stat drain hostile elementals of their element? Maybe for some sort of Con buff?
So your Fire Kineticist is usually blasting, but when the group encounters a Fire Elemental, he can't hurt it, so he starts siphoning off its flame, weakening it for the rest of the party to kill it and then he gets a little buff after that.
It demonstrates his mastery of that element but he can't use the same tactic for every fight.