Love the new spell system, and thinking about it today had some ideas, which may or may not be new to you.
I had an idea that might work well for a mid to high level mind, spirit, or life spell.
Compassion-gives unit the mercy promotion.
Mercy promotion--gives unit a 100% withdrawal rate when attacking living enemies. The unit would withdraw just before finishing the enemy, if possible, leaving it alive with ~10% HP. Would have no effect when defending other than that it would have a 50% to be removed after surviving an attack. Perhaps some strong good units could have this automatically as a balance.
Also some possible Air/misc spells (since I noticed none are in yet)
Guarding Winds-Unit(s)ignores (enemy) first strikes until the after the next battle.
Suffocate-Damages all living units in the tile by a medium level. (counterpart to banish)
Tornado-Prevents units from entering or working square for 1 turn. If cast in desert, destroys any improvements inc. roads. If cast on an ocean squarehas 50% chance of surviving to next turn and moving to random tile, possibly destroying any ships there.
Also, have you considered having spells/summons that require more than one unit to cast? It's quite in the spirit of fantasy to have a little council of mages required to cast strong spells, maybe summoning a titan/esper whatever or global effects such as +1 food in all cities for 1 turn. If a summon, make it a world unit perhaps.