[Saxon]

Nice ideas!

How about some more weather spells, perhaps linked to nature magic as well.

Driving Rain

Powerful rain lashes from the sky, flooding fields and knocking down crops. 50% gold income and supply reduction in targeted province this turn.

Gentle Rain

Kind and blessing rain caresses the land, giving substantial help to crops in your dominion. 25% gold and supply increase in all provinces with your dominion.

Mud Patches (combat)

The mage brings forth small springs from the ground, creating patches of mud which greatly slow movement. All troops entering these square lose any remaining movement this turn. Each point of magic produces one square of mud.

The Last one is aimed at those low level water mages who cast quickness and can then do nothing in battle. A little bit of mud will not change things too much, but will slow troop advances and flight. A low level mage can not do too much with this, but a high level one would mess up a field quickly.