Either an extra X% (for any spell that has a numerical effect, including haste, slow and heal, drain life spells, possibly excluding despair since it is so powerful already) every 1, 2 or 5 levels, or extra shard power for spells of all types (maybe 1 every 5 levels, as an example, probably needs tweaking).  This would give an incentive to go Path of the Mage, and give more importance to Spellcaster experience.  I think Path of the Mage heroes could do with a bonus for experience levels on top of what regular non-mage magic users get.  Could also give certain bonuses based on Warlock or Summoner.

Should be quite simple to implement, and you could provide an XML tag that would allow modders to find a good balance from the base files of the Beta5-last patch, if the base values need a bit of tweaking.

What do people think?  Devs, Derek, what do you think?

Look forward to your replies.