I recently had the idea of gaining some bonus to spells of a specific school of magic provided that you have an appropriate treatise in your offhand as you cast the spell. This would be relevant to spellcasting builds that use wands or one-handed weapons.
I think that the main drawback for such a mechanic would be that you're not using your offhand for a shield or other weapon, and you also can't use a staff and gain this treatise benefit. It would also necessitate that treatises have some kind of durability or limit to the number of uses they have.
I think there are a fair few questions that come with this idea. Availability, scaling, and durability.
Availability - I think that this should be a tier 2 or 3 passive magic mastery skill, or require having tier 2 or 3 sorcery or magic mastery skills unlocked to work. I'm aware that there is no space left at tier 2 in Magic Mastery, and it makes me question whether this idea would work at all. Treatises would also need to be read before any benefit can be gained from them as well.
Scaling - I think that the tier of the used treatise compared to the tier of sorcery skill used should impact offhand treatise effectiveness. For example, a tier 1 sorcery skill would get a major increase in effectiveness as a result of a tier 4 offhand treatise. At the same time, a treatise of a lower tier than the cast spell would have no effect on the spell or its cooldown. I think the main benefits should be a linear decrease to backfire chance and cooldowns, and an increase to accuracy.