There is also the problem that sufficient skill in enchanting allows you to cast your spells for free.
This leads poor mage players to despair, wondering "what's the point of investing into magicka?"
Well we can solve both issues in one fell swoop: make it so that each point in the magicka stat improves every spell's effect by a certain percentage (say 2%, this way if you're level 51 and invested purely into magicka you get +100% spell effectiveness).
This way, we solve the problem of illusion having a hard time affecting higher level-enemies, and destruction damaging higher-level enemies. We also give a tangible reward for investing into the magicka stat.
What do you guys think?

