Hmm, Vampire Lords do have summoning now. I'm pretty sure thats new. I'll have to play as the Calabim next.
I too would like more spells, probably with some restrictions. Perhaps different spells could be available to units of different civs, different religions, and different (owner's) alignment. For instance, Evil-only Spirit spells like the ones Laroth used, like causing unhappiness / a chance for revolt in rival cities, and sacrificing units to forge their souls into mystical weapons (probably equipment of different qualities based on the units sacrificed)
I also wouldn't mind if there were some way that individual mages would just happen to have different styles of how to use magic, maybe having several different spells with the same requirements, only one (or maybe a couple, for arcane civs) of which an individual unit could actually cast. Of course, that would probably be very inelequent to implement, and likely not worth it.
Currently I think the worse spell in the game is Nature III Divine (Entangle). Its effect is identical to Sun II Sorcery, except that it only lasts for 1 turn. This means that the units inability to move actually wears off before it is that player's turn to move anyway. One turn of immobility has absolutely no effect.