Short answer: read Jack Vance's 'tales of a dying earth'. It's the reason dnd magic is called 'vancian'.

Longer answer: in that series, magic works by just remembering words, and then saying them. But these magic words are powerful things, weighty in the mind, hard to carry. And, when said, they tear themselves out of your mind, causing you to forget them.

So, not 'spell slots' per se, but the idea is you're prepping spells almost as a 'potion', something you carry in your mind, and consume to cast out a spell.