The warlock spell system in NWN II certainly does work, 99.8%. There are no slots/day, every spell can be cast as many times as you want, even the high level ones. Unlimited invisibility, darkvision, charm person, high level shapechange into a cornugon, etc. Very much like Chainmail. Try it! I think some sort of combination of that and the ritual casting from 5E to balance out the non-combat functions would be cool.
