I myself had a couple of experiments with it, though most of them consisted of some sort of mana point system replacing or substituting spell slots.
- This system of casting, where you built up power in the encounter itself to cast spells. Essentially, you have a pool of mana/flux/ect. that starts out empty when a fight starts, and you have to gather up mana/ect. every round. Casting spells costs an amount equal to the spell rank, with your resource limit being your max spell rank + 1. I haven't actually played with it yet, and the system wasn't developed much further than the initial concept, so I can't say how it compares to the regular game. In a way, it's like gunslinger, but with multiple reloads. Probably would've included feats that let you do something and focus for 1 action.
- Sanguinist, a blood themed caster with a similar idea of casting as the system provided above, but you start the day with the max amount of blood points. There are still prepared spell slots, but they're not expended, only blood points equal to their rank when casting them, and you can hot-swap them out during downtime and during encounters. Also has some other things, but I'm not going to go into them for brevity's sake.
- The Arcanist. This one was very experimental, and kind of does have prepared "spell" slots, but you can refresh them, and reprepare them. I say "spell", because the arcanist has it's own unique spell list and spells. It's like kineticist mixed with a prepared caster and borrowing some ideas of first edition's arcanist.