Honestly, I'm against making overmap mission spells weaker - they're already much less worthwhile than crafting spells (due to rarity also being linked to duration) and generally you'll have enough crafting materials along with the fairly abundant uncommon spellgems to afford to craft (espescially if you have bothered even slightly with citybuilding, since then crafting material survivor missions are borderline free resources). I don't bother with the overmap missions for spells as a result, only doing those that give a building I want as well. It might be worthwhile making the missions that have building riders give common-with-no-extra-level, and then those that only have a spell give +1 level uncommons to make those more worthwhile as well.
