On Vancian Magic
When writing my magic system for Beasts & Barrows, I had two goals: keep it simple and somewhat logical. Towards simplicity, I reused the existing game mechanics: 2d6, attributes and level. I also considered using Necropraxis' fabulous spell dice but simplicity won out.
Making the system logical took a lot more thought.
Several people describe my magic system as Vancian (ie. spells must be prepared daily; have a fixed purpose and finite usage). I did not set out to make my system specifically non-Vancian. This is good because when I think about it, it really is quite Vancian... but with twists.
First twist: preparation. It's silly to expect magi to prepare spells every day. It does make sense that spells are learned, practiced and mastered. We see this in Harry Potter; the kids all practicing enunciation and wand flicking. But this leans more toward practice than preparation. It's also logical that magi have a limited number of spells they've mastered. This number increases as their experience increases. I want players thinking of creative ways to use the spells they have.
Spellbooks. What dolt-of-a-mage cannot memorize a few spells? Must they write them all down in a book? If they're without that book, can they no longer cast a spell? Lame. No. But books are quite useful. Magi can read and cast unfamiliar spells from scrolls and books, but it takes more time (a turn). It also incentivizes scroll hunts. Fun!
Which leads us to usage. It's ridiculous that a mage would forget a spell after casting it. Same goes for it vanishing from their spellbook. No. That's crazytown. The twist here is that casting spells takes a physical toll. A mage can safely cast spells, based on their stamina (con), and have it scale with their experience (hd). This could have also been level but I feel like hd is a bit closer to health... same diff. Cast all the spells you want... but it will eventually hurt.
Lastly, we have spells with a fixed purpose. Spell authors will have a lot to say about this. But a few twists can keep them simple and allow for more creative usage. I love levelless spells. I can see an argument that spell level denotes complexity or whatever... but I prefer to let a mage learn whatever spell they like. I also added enhancement spells. Spells that enhance a target spell but cost an extra spell slot. This helps keep spells simple and flexible.