I’d favour a system that limits magic in a less predictable way than a set number of max spells, ie some sort of dangerous “overload” mechanism that makes heavy reliance on magic a foolhardy enterprise. Some “game within a game” aspects (mental duelling between opposing magic-users, deflecting enemy spells back at the enemy or absorbing their power, and so on) would be alright if it doesn’t go too far. Magic items would be useful to that end, but the sheer amount of items in WHFB (at least back in 5th and 6th edition when I was playing) is too many for my tastes.
I’d want some variety in spells between magic-users of different allegiances/”themes” so that not everyone is chucking the same fireballs at each other.
If a ruleset purports to be generic and customisable, I’d want it to let me “modulate” the degree of manifestness or subtlety of magic to fit the setting I’m going for. For one setting I might want wispy, intangible-seeming magic, and for another I might want teleportation and lightning bolts.
All of that together is a big ask, I know. It’s my hypothetical ideal ruleset I’m describing here. In practice I can settle for less