I pretty much never want a mechanics-first system. That's the opposite of how I want TTRPGs to work.
The way I did it in Elements of Magic back in, oof, 2003 was that each level you got a number of magical effects you knew how to create, each of which had a Magic Point cost. And you could build spells on the fly, but doing so took two whole turns to cast the spell.
However, you could design a number of signature spells - premade combinations of effects, ideally with an evocative name - which could be cast with a single action.
So if you knew evoke force and evoke fire and abjure outsider, you could make a bunch of different variations on 'shoot fire at someone, knock them down, and if they're a demon, and hold them in place.' But you'd probably have some specific spell you'd call, "Demonbind."
And on top of that, each magic user was encouraged to take a Magical Tradition, which made a thematic suite of certain spell effects cheaper, or gave you new ways to gain MP to cast your spells. So like a Blood Mage could discount a spell if they had done slashing or piercing damage to someone in the past round, and if you were an Animist you were able to summon spirits and communicate with them more easily.
I admit it was somewhat clunky, but I'm 21 years more experienced now. I think I could take that premise and make a more polished version today that would encourage players to have evocative themes for their characters, while ensuring that the actual game mechanics were easier to keep track of.