Spell slots and forgetting spells has always been one of my least favorite parts of D&D. I respect the origins, Jack Vance’s The Dying Earth series, and the limitation works for mechanical reasons, but outside that particular point of reference, this mechanic doesn’t make much sense. However, I recently reread the Dresden Files series by Jim Butcher, and there was some ideas in that series that I thought improved the feeling of Vancian magic without changing the rules.
I Call it Spell Weaving.
Spell weaving provides an alternate explanation for spell slots and the daily "preparation" of spells, while providing a better explanation for how one can prepare the same spell twice, or how a spell could be prepared at higher levels (if that’s a thing your game does).
On the Process of Spellcasting
Historical spellcasting was a particular art, Each spell took up to an hour to cast, required absolute concentration, and most spells were of a utility variety, given their lengthy cast times. Striking down your adversary with a bolt of lightning was less appealing when they could stab you and hide the body before you finished the preparations.
This changed with the discovery of triggered spells. One could do all the preparation and casting ahead of time, but leave the spell in a perpetual loop near the end that could be undone and completed when the time came. Suddenly, combat magic was now feasible, as a wizard could prepare their bolt of lightning at home, then release it when such a need arose.
Limitations of Spell Weaving
There are two major downsides to the process of spell weaving. The first is in regard to the time a spell can remain prepared before fizzling out, roughly one day, with sunrise being particularly harsh on an incomplete spell. The second is the physical toll of a prepared spell on the caster. A caster needs to bleed energy into the spell constantly so it is ready for casting at a moments notice, but it also takes the full amount of energy needed to cast the spell traditionally on it’s completion. This results in delayed casting taking much more energy overall than the traditional methods.
Spell Weaving changes none of the default assumptions of casting per the B/X rules, but does provide (at the discretion of the Referee) the possibility for "Ritual" casting if so desired. Ritual casting requires 10 uninterrupted minutes of focus per level to complete and still consumes a spell slot, however, a prepared spell slot can be "released" and the used for a ritual spell upon completion.