Spell points emulating energy
I'm not sure that this would actually be fun to play, but I wanted to get this idea down somewhere that wasn't a twitter thread. It would probably be too much bookkeeping for a TTRPG, but maybe it would work well for a video game?
The main impetus is making spell casting feel natural. In real life, you only have so much energy to do things, whether they be physical or mental tasks; you get tired as the day goes on, eventually you go to sleep and hopefully wake up refreshed, or you push yourself too hard and don't wake up refreshed. If you push yourself too hard you start making mistakes, or get injured, or something.
Like with most point based systems, every spell costs a certain number of points to cast. Complicated or powerful spells are more tiring and require energy and focus to get right. And then there are cantrips, which are the easy spells you could do in your sleep, you could do these all day. And you can cast any spell you have the points for. Maybe there are ways to get extra points - push yourself too hard, at a cost; maybe risk having the spell go wrong; maybe some divine boon or magical effect, maybe some kind of potion. But as you level up you don't get more points, spells just get easier as you get better at casting them. Similarly, when I get better at something I don't get more energy, it's just that I learn to do things faster, more efficiently, with less stress, with less energy.
Maybe at level 1 you have 5 points. Tier 1 spells cost 1 point: after casting 5 you need a good night's rest. Maybe you can regain 1 point by taking a short nap. Maybe Tier 2 spells cost 4 points, so doing one mostly drains you for the day. Tier 3 spells cost 9 points. This is way more points than you normally can cast. It might not be totally impossible to somehow get all those points, but it should be rare and dangerous, a heroic last ditch possibly fatal effort, a Gandalf vs the Balrog type moment. Or, less dramatically, this is kind of like the Stress mechanic in blades in the dark.
And then at level 2, tier 1 spells cost 1 point still, but tier 2 spells now cost 3 points, so you can pretty reliably get two off if you just take a nap in the middle of the day. Tier 3 spells now cost 6 points, so you'll have to figure out how to get that extra point, but one Tier 3 spell a day is now in reach. Note these spells will have to be balanced a bit differently than in D&D.
At some point, tier 1 spells now cost 0 points, they have become cantrips. You don't have to worry about scaling cantrips with level now as well.