(sorry for the brain dump)
Changing how magic works, without changes the underlying mechanics (spell slots, memorizing spells, etc)
A lot of people have issues with Vancian magic, but instead of shelving it I thought about changing what it meant and how it worked.
All spells are rituals; higher level spells take longer to prepare and cast. (A level 3 ritual might take 10 minutes to cast, a 9th level ritual might take hours to perform).
All rituals are 'dealings' with otherworldy forces; demons, spirits, elementals, etc. The ritual is a means of contacting the entity and making a deal; the mage provides something in exchange for a favor. A pinch of sugar might convince a spirit to hide you from sight, fresh blood might grant visions or insight.
A wizard can attempt to perform ANY ritual they know, regardless of level. No limit # times per day (as long as they have the time, and the means to perform the ritual').
Low level rituals might just require a waved gesture and a muttered incantation, allow them to be used over and over 'in the field'. High level rituals require carefully drawn circles, special candles, implements and might even be limited to being cast at special places or times (by the light of a full moon, in a graveyard, in shadow of a oak tree, etc)
Some spells are better off being cast 'on the fly' (Fireball isn't very useful when it requires a 10 minute ritual cast over a fire burning in a brass brazier); experienced wizards can 'store' rituals (referred to as "spells") upon completion, unleashing them later.
Spells represent a deal made with an entity to be fulfilled later. Spirits are short minded and experience time differently, so it is difficult to hold their attention and to get them to agree to a delayed bargain; only experienced mages can bind spirits in such a way (Hence, spell slots). Once a spell has been released a new ritual can be performed to fill the empty slot (unlimited times per day, no requirement to rest).
A spell might require an implement to contain the magic; a scroll, a wand. a token that is crushed or burned releasing it's power. Some spirits are fickle and may demand some actions to be taboo; perform the taboo and the deal is broken; you lose the stored spell (or a spell in progress is suddenly cancelled). For example; the spirits that hide you from sight hate violence; draw the blood of another creature and forfeit your invisibility.
With this system "utility spells" can be cast over and over (these spells are often overlooked for damage spells in most games, as they are so situational), and a wizard's combat ability isn't really increased at all. Wizards can can an unlimited number of spells in a day, even "memorizing" spells over and over, but at the higher levels it could take a long time to 'fill' all of their spell slots, so high level combat spells become precious.
Limitations on rituals and bargains make magic feel more "magical", prevents spamming of high level magic (Can only perform Animate Dead by the light of a Harvest Moon, or something) and introduce the weirdness that is common to mages in stories. A wizard who has prepared Invisibility can't draw blood maybe, a wizard who has prepared X can't use iron tools, or can't speak lies, etc)
These bargains can also be exploited in enemy mages. Rival wizards might have different limitations on the same (or similar) spell, so recon and research might give clues to their weaknesses ("Aha! Melgon the Black has a copy of the Book of Names, and is fond of summoning Elementals! If he's using the Book of Names as the source of his summoning magic, he is forbidden to consume meat or blood! If we mix a few drops of blood into his wine he'll be rendered powerless!"
A smart mage will hunt down multiple versions of the same ritual; the assassins of the Order of Shades can obviously draw blood while Invisible, but their ritual is carefully guarded...