One of the key mechanics of D&D is Vancian spellcasting where the spellcasters have a certain number of "spell slots" that they can prepare in a given day. Magic Users (the precursors to wizards, warlocks, and sorcerers) would "memorize" spells and the Clerics (and later Paladins) would "pray" for spells. Later, druids and rangers would be added which would also pray for spells.
This was one of my biggest stumbling blocks for getting D&D magic because it didn't make sense to me that you just stopped knowing something because you used it once. And while I get the idea of becoming tired, the spell slot system was too rigid to represent how exhaustion works.
This is called Vancian magic because it is based on the Dying Earth novels by Jack Vance. None of which I've read, which probably contributes to my just not meshing well with Vancian casting.
In the series, magic was released by using magical words and the human mind could only hold a limited number of syllables in mind at a time and using them would wipe the knowledge from their mind. So the wizards would have to study their tomes everyday to regain their ability to perform magic. This mechanic was carried over to priests save that they would begin the day by asking their god for specific miracles. In both cases, the spellcaster needed to pick specific spells and couldn't change over to different ones if they decided that they didn't need a prepared spell after-all.
Raymond Feist, the novelist who created the Midkemia setting and wrote the Riftward novels, did some earlier work on D&D and refined this narrative into the idea that spells were long rituals but wizards had eventually built in a cutoff at the end where they could leave a spell unfinished save for a word or two. They would then have that spell basically just hanging on their word to release. Mechanically similar, though the narrative is different.
I have read some of that series and his approach to Vancian casting was the first one that made some sense. It accounted for why you "forgot" a spell. You didn't, you just expended the ritual and didn't have time to set it up again. And it explained why you had to prep spells ahead of time. Again there would be no time to do the ritual on the fly. However, it still didn't account for why the spell slots were rigidly structured on a level basis. And I still felt that surely you could do something with all that raw power even if you couldn't shape something intricate.
What finally made the idea of magic slots work for me ... sort of ... is the video game Nioh 2. But even here there aren't slots, there's a "capacity" and each technique uses up a certain number of points of that capacity. But that's not the big "ohhh" moment for me here.
In Nioh 2, your Onmyou (and Ninjutsu) are physical tools and talismans which are consumed. So you would go to the nearest shrine and choose to "Ready Jutsu"
This would bring you to a menu show your capacity and what skills you've learned.
And then you could assign those pieces of gear to one of your eight hot-key slots. Resulting in me having a bunch of stuff I want available immediately and some items I use between fights so I can take my time about activating them and thus don't need to use one of my limited hot-buttons.
The main thing that made me go "That makes Vancian magic make so much more sense" even more than Feist's rituals with the cut-offs. Is that each of these tools is a consumable item and once you use it, you have to make another one.
The energy isn't something you have on tap where rituals are used to funnel it. It is something you take effort and time to create and then you expend it like a grenade, potion, or ammunition.
On top of this, Nioh 2 does have powers that you can use repeatedly over time as power builds up in the form of yokai powers and yokai shift. Both of which hinge on your nature as half-yokai and the yokai spirits that have allied with you. For these powers you have two pools of endurance that build up over time, one of which you can use piecemeal to summon your yokai powers and one of which you have to build up to a specific threshold in order to shift into a yokai form.
So they both have consumable one-use spells in the way D&D does in the form of Jutsu and inherent magical energy to fuel powers in the form of Anima (fueling yokai powers) and the Amrita Gauge (the threshold for changing into yokai form). But they are separated.
I'm going to set aside Anima and Amrita because they are basically "Sorcery that makes sense" and function by getting rid of spell slots.
The Jutsu is the method I'm focusing on here again because:
Each prepared "slot" represents a tool prepared using engineering, chemistty, mystical reagents, or appealing to spirits.
The stuff that makes the tools and talismans is external to the character.
The slots are not specifically numbered. As a note, point pools have been an alternative magic casting method since late 2nd edition.
This combination of creating a physical consumable and dropping the level-sorted slots was the thing that made Vancian casting make so much more sense. But then again... this wouldn't be Vancian casting, would it?
Side note, another facet of Nioh 2's "Jutsu" system is that leveling up the skill does not improve the damage that technique does. Fire Shot I, Fire Shot II, and Fire Shot III all base their damage on your Magic rating and other inherent bonuses.
What leveling up the skills does is increase their efficiency and increase the maximum number you can craft. For example
Fire Shot I: Prepare 1-6 fire shot talismans at 1 capacity each.
Fire Shot II: Prepare 1-8 fire shot talismans at 0.8 capacity each
Fire Shot III: Prepare 1-10 fire shot talismans at 0.7 capacity each.
This is largely just neat and not part of my epiphany on Nioh 2 and Vancian casting.
Now, you can't just change D&D to match that. It would be a completely different flavor of magic. But it is curious to see how it things would have developed if they had taken this track.
The big change here is that this mode of "magic" is more akin to being a D&D artificer than a D&D wizard. As you can also see, it would also have worked with some rogue abilities too with craftable thrown weapons, grenades, and medicines.
My suspicion is that if D&D had started with the assumption of wizards and clerics preparing relics and talismans between fights then it would be the sorcerer that was a late comer rather than the artificer. Though the artificer might not have been called such since it would be the base assumption.
I kind of suspect if the sorcerer had been introduced in that environment we would have heard something akin to the "no sci-fi in my fantasy" we get with artificers today but instead we'd be having "no mutant superheroes in my fantasy" with sorcerers.
