If you've seen black clover you kind of get what I'm talking about.
It's a huge task to create the spells themselves but even a framework for the grimoire itself would be awesome and leave the spells up to other modders.
"A grimoire is a textbook of magic, typically including instructions on how to create magical objects like talismans and amulets, how to perform magical spells, charms and divination, and how to summon or invoke supernatural entities such as angels, spirits, deities, and demons."
So idea: A Grimoire book (that could also be modular for other modders??), depending on the page that's open is the spell that's casted when the activation button is hit - Meaning pure mage builds!
If the grimoire is selected in the spell wheel, it'll float around open and ready, can move it around if it's in the way for whatever reason. - Of course this seems clunky but I think it could work for some people.
of course I don't have the skill to do this myself lol but this is just an idea/suggestion