Since we're talking magic/casting systems, one of my favorite I've played with is from a system called Cloudbreaker Alliance. Spells operate on MP instead of spell slots, which I like. Its mechanic for learning/leveling up magic is also unique in my experience; if this copies a pre-existing system, I've never encountered it.
Magic is split into elements (6 for arcane, 3 for divine) and you grow in breadth of magic by getting more element specializations as you level up. Each element has a general focus (Wood is piercing and ranged damage, water is healing, earth is terrain and defensive buffs) but you get separate spells for each combination of elements (fire and wind lets you do AoE fire damage, metal and earth for a full suite of combat buffs, etc.) and having sets of 3 and all 6 elements grant you entirely new elements of spells. Divine magic has a similar system, but less extensive since it's just 3 elements. Individual spells also increase in power with character level, so it's still useful to be a partial caster or a gish since your single-target fire bolt will still progress like a full caster's would (though full casters get separate bonuses and skills related to casting). There are both battle and utility spells and spells used in combat involve rolling against a monster's spell defenses for partial or full success. And you can use in-game time to re-assign your elements if you feel what you've got won't meet the upcoming situation.
It's probably the most fun I've had playing a caster in any single system because it requires active participation in combat and rewards foresight & planning while remaining distinct from what martials do without casters just becoming a god partway through the campaign.