One of the elements of D&D that's never felt quite right to me is so-called Vancian magic. The idea of impressing a spell formula onto one's brain, and then forgetting it utterly as soon as it is cast, goes strongly against my conception of what magic and wizards are and how they work. The model most ingrained in my mind, from fairy tales, folk lore, and fantasy fiction, is that a spell caster simply knows a spell, and may cast it whenever he chooses. It might cost him something in the way of personal energy or magical reagents, but the idea of forgetting what he knows each time he casts feels rather...disassociated. I'm no sage of Gygaxian lore. I don't know what his thought processes were when designing the system, but it feels very much like an effect-first mechanic, designed specifically to limit the number and power of spells a magic-user can cast, fluffed with the trappings of Vance's system. I'm not knocking Jack Vance, whose works I've never read; I'm sure his rationale for magic is a fine fit for his particular milieu, but it's a specialized vision, unique to that setting, not an embodiment of classic fantasy standards.
The most frequently mentioned alternative, at least that I'm aware, is a spell point or mana system, with a pool of power representing the caster's personal reserve of magical energy. He doesn't forget the spells as he casts them; he simply expends energy. It's still a bit artificial, in that a daily cycle is superimposed - no matter how much or how little power a caster expends, it takes the equivalent of a night's rest to replenish it.
I have in mind something a little different.
Spells are cast using both sonic/verbal and material components. Verbal spells are the foundation of spell casting. A caster can cast purely verbal spells at will and without limit. Just like a song or a poem, he knows them, and won't forget them under ordinary circumstances. However, purely verbal spells are sharply limited in their power, with only minor or trivial effect. To focus, refine, and amplify their effects, spell casters employ mixtures of rare herbs, minerals, and essences. The more powerful the effect desired, the more rare and exotic the materials needed. The formulae for the material portions of spells are more complicated than the verbal components, like recipes which are best prepared ahead of time by carefully following the instructions from a spell book. Wizards often wear voluminous cloaks and robes, in the myriad hidden pockets of which they keep the vials, pouches, and ampules of prepared spell components.
Each spell in the rules is based upon an appropriate verbal-only spell, i.e. a cantrip, which by itself produces some trivial effect. More than one spell might share the same base cantrip; for example, burning hands, fire ball, and delayed blast fire ball might all be based on a cantrip that conjures a spark just hot enough to light a lamp or ignite dry tinder. Cantrips may be cast at will, requiring nothing but the caster's voice. Any spell of 1st level or higher requires a small quantity of prepared materials, which are consumed or destroyed in the process of casting the spell. Material components are expensive, and the cost increases with spell level.
1st - 50 gp 2nd - 100 gp 3rd - 200 gp 4th - 400 gp 5th - 800 gp
6th - 1,500 gp 7th - 3,000 gp 8th - 5,000 gp 9th - 10,000 gp
These costs are only tentative, and might need to be adjusted. Certainly they can be adjusted upwards or downwards for particularly high or low-wealth campaigns. Weight of spell components should probably scale with spell level also, perhaps 2-3 coins of weight per spell level. Time taken to prepare each dose of a spell formula could be as little as one turn per spell level, or as much as a day per spell level, depending on the flavor and needs of the campaign.
Material components have no effect unless used properly and accompanied by the appropriate base cantrip, thus limiting their use to those trained in the ways of the arcane. Exactly how material components are used to cast any particular spell is up to the player and DM - they could be swallowed in a particular order, tossed into the air simultaneously with the speaking of a crucial syllable in the cantrip, crushed, burned, used to draw runes, or whatever else might be imagined.
You might create a detailed list of the specific components needed for each spell, perhaps naming 1-3 items for each spell and assuming that any additional ingredients are common and easily obtained. If that's too much effort and bookkeeping for your taste, you can hand-wave specific ingredients and simply assume that whatever materials are needed are acquired by paying the cost of preparation. The important point is that a magic-user must prepare materials for specific spells ahead of time. If an enchantress wants to be able to cast sleep, shield, and/or magic missile on an adventure, she'll need to prepare at least one dose of each specific formula. Once a dose is prepared, it is good only for that particular spell. Preparing a spell formula in a properly equipped laboratory is automatically successful. Preparation in the field is less certain. An Intelligence check is required, possibly with penalties for particularly adverse conditions or inadequate equipment. This should be made in secret, with only the DM knowing in advance whether the spell will function when cast.
It's easy to restrict more powerful spells to more powerful magic-users by applying the original spell progression charts. The level that a Vancian caster would gain access to a particular spell level is the same level at which one in this system would attain the skill necessary to formulate the material components and successfully blend them with the invocation of the cantrips. Thus, a magic-user still can't cast 3rd level spells until 5th level of experience. The number of spells usable per day, however, has no application here.
If stockpiling of spells is likely to become a problem during down time between adventures, you could rule that the components, once prepared as specific spells, have a limited shelf life - a week, a month, or whatever is necessary to keep things under control. In all likelihood, if you aren't handing out ungodly heaps of treasure in the campaign, I think the problem will largely self-regulate.
A magic-user is no longer limited to a hard-and-fast number of spells per day, but by the same token, can no longer just regain full power with a night's rest. Resource management becomes a long-term concern rather than day to day.
The number of spells a magic-user can carry on an adventure depends on
his wealth, prep time, and encumbrance more than on his level.
Magic-users will need to spend a good deal of their loot on spell components, which will tend to reinforce their reputation as studious and anti-social personalities. This isn't a hard and fast rule, obviously, but it will tend to push magic-user characters in that direction.
There's a strong disincentive to cast spells for frivolous reasons, with the obvious exception of bare cantrips.
A magic-user now has something to spend his or her starting money on besides buying all the miscellaneous equipment. If desired, the character's mentor may provide a one-time boon of one or two spells' worth of material components.
There's a strong incentive to collect rare chemicals and reagents on adventures. If you don't assign specific materials to each spell, they can just be totaled by value.
Even if you don't make each and every spell require specific reagents as a general rule, it's possible to drastically restrict the use of certain spells by assigning some exceptionally rare component to them and making those components unavailable for purchase. If a magic-user really wants to cast that spell, he's going to have to quest for the legendary pyromycofusium mushroom that only grows on red dragon corpses.
There may be more implications that I've missed.