The amount of points you are allowed to put into a skill should be limited in some minor way by either character level or stat point to force some skill diversity.
Damage for spells should be based off of skill level and int just like melee damage is based off of skill level and str.
There should be a higher int and wis stat level required for the high level spells. A simple +stat item will allow you to learn on spells with the base stat points. Currently it is too easy to be a warrior and with little sacrafice be a decent spell caster too. Starting stats plus skill points from trainer gets you most of the way for low to mid level spells.