At time of summoning, a spirit's elemental abilities are assigned an appropriate element - for now, that's mostly gonna be Electric for Air Elementals, but come the Magic book things like Blast would certainly be an option. For Earth, sticking to current stuff, there's Acid; with the splat, stuff like Metal or Sand would easily work.
It's not unlike how Water Elementals are stuck with Cold for now, but Water and Steam will become options when they're added back in.