Spellbook templates. Not sure if this is a feasible thing (haven't seen it before), but constantly managing spells is tedious. I would love to be able to save multiple layouts for situations, ie: offensive spells, party buffs, solo buffs, etc.
If you'd want to add flavor to it, I'd even go as far to suggest that it be implemented as an item "Spellbook" or "Tome" or w/e that you can save/load the templates to.