Independent Uses:
- air can be used to levitate, mimicking flight, without the need for wings
- mental manip can be used to wipe memories, implant suggestions (like inception without the need for dreams), and manipulate the intensity of someone's emotion.
- Intangibility works as a nifty way to give your character a 'phantom limb' that they can see but not use for anything effectively.

Combo suggestions:
- water and air can work together to make a mini storm over your characters body. Literal raincloud when sad, hmmm?

plant power?
This is included in the earth manipulation to "be able to grow a single flower" but maybe there could one similar in cost that alows them to grow a lot more smaller plants? To help grow larger plants in incidents. (and feeling drained if they grow too much) if used together with water and earth elementals.