Is it even possible? I'm been trying to wrap my brain around it for a while. The idea is that prepared casting would stack and spontaneous casting would stack (such as for Paladin/Cleric or Druid/Ranger). In my head I've got that there are Full Casters (Clerics, Druids, Wizards, Witches, Oracles, Sorcerers), 3/4 Casters (Bards, Alchemists, Inquisitors, Magi, Summoners), and Half Casters (Paladins, Rangers). The idea is to have them share a list and grow off of each other rather than have separate weak lists. I know I could make up some sort of point system, but I wanted to know if someone had come up with a Vancian system first.
Well, you already named the problem, they need a shared list. Since 3/4 and 1/2 casters get "early entries" this is quite a problem... removing them weakens them, letting them stay strenghtens the full casters even more...
Also some spell lists are theme based, like the Bards's illusion and healing magics...
But in general, assuming shared lists, it should be not that big problem, we introduce "magic level points" and a table that relates them to spell levels (and possibly spells per level if you don't want slots per level as additional thing). Full casters get 4 points a lev, 3/4 casters 3, half casters 2. Also you could introduce an amount of slots per level and these slots become highest level slots (or something like "you get 1 slot of the highest level you can cast and one slot two levels below that")
here's the table (only for , using max level) the wizards progression:
spell level____points needed_______full cast gets at__3/4 g.a.__1/2 g.a.
level 0: ______3 p_________________________1_______1________2
level 1: ______4 p_________________________1_______2________2
level 2: ______12 p________________________3_______4________6
level 3: ______20 p________________________5_______7________10
level 4: ______28 p________________________7_______10_______14
level 5: ______36 p________________________9_______12_______18
level 6: ______44 p________________________11______15
level 7: ______52 p________________________13______18
level 8: ______60 p________________________15______20
level 9: ______68 p________________________17
Yeah, as a result Bards get lvl 8 spells. It's basically a logic consequence of merging spell lists.
Another idea for such a system: one could rule that magic points can be spend for other stuff, for example: remove lvl 0 spells from that list and say "you need to spend 2 points to get access to lvl 0 spells"
Or wizards start with a big bunch of them and every magic school needs to be bought first (solving the bard's special list problem - bard's don't get that many points so they have to decide which schools they want, usually illusion, enchantment and "healing" as a new school. just like weapon profiencies every class gets school profiencies, which means bards without a dip into wizard are unable to buy the evocation school for example, however unlike wizards they have the healing school in their "profiency" list - if you introduce such a system, individuality of casters is also raised, especially for special gishs like wizard/rogues who use illusions and shadows and basically nothing else)
I'd still prefer getting rid of the vancian system for a magic point based system as nearly every other game featuring magic has, to name only one advantage: a mage could decide if he wants to spend his magic points to shoot hundreds of small fire bolts over the day, one at a time, or gather all his powers for one big shot that shatters walls (PF introducing the idea that lvl 0 spells are not spend is a direct result of the lack of weak but economic spells)