Sure sure, Ejecta Barrier might be overpowered. I didn't say it does a lot of damage, just that it does damage to enemies crossing over, but there's lots of room to tweak it. Maybe the idea of Channeling spells could work.
I like some of those other ideas. Evaporate is a good counterpart to Make it Rain, those spells should have exactly the opposite magnitudes and effects.
A lot of these require dual-casting. How would that work... oh, right. If you can assign a spell to left-click and right click and click them together... I think that would be neat. Maybe hide most of the combinations and keep them secret until you actually cast them the first time. The manual could have a tip on one or two of them (and note that casting Make it Rain and Evaporate at the same time does nothing), and encourage the player to experiment.
Dual-casting would cost a lot more mana though, since it's two spells at once, but there might be a Talent or Trait to reduce the cost a bit.
