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.
The NWN2 Warlock works because it only ever has a very short list of spells selected from a very small pool of spells. I wouldn’t want every class to operate like that. It would be ruinous to what makes clerics and wizards special.
The 5E spell slot system works great. It is functional, flexible, more easily balanced than a mana point system, easy to comprehend for new players, and demands players use foresight and planning to gauge their resource management. It allows a certain degree of rule homogeneity between all the various caster classes while also affording them distinct identities.
I’ve been DMing for about 25 years. I did not like the spell slot system from previous editions of D&D, but 5E nailed it.
