Dual Preparation Casting (3.5e Variant Rule)

Dual Preparation Casting
One of the problems with prepared casters is that they have to prepare such a small list of spells, yet a number of situations can come up that make them unable to respond properly to what's ahead. For example, a wizard could prepare Dispel Magic... or Fireball, and would then be unable to be effective against buffers or swarms of creatures. This variant allows them to gain a little versatility at the cost of having their usual number of spells for the day.

Mechanics
Upon preparing their spells, for each spell level a prepared caster has they choose an even number of spell slots. For every two spell slots chosen, they immediately expend a spell slot of that level as though they had cast it already. In the chosen spell slots, they may prepare two spells, and if they use either one, the spell slot is expended.

For example, a wizard could choose two of their third-level spell slots to make into dual-prepared spells. They immediately give up one of their other third-level spells for the day, and can prepare two spells in that slot, for example in the first they could prepare Haste and Displacement, and in the second they could prepare Fireball and Dispel Magic. If they were to cast Fireball from that spell slot, they would lose the ability to cast Dispel Magic from the same slot.

Alternate
An alternative would be changing how prepared spellcasters work entirely. If you choose this option, instead prepared spellcasters treat all of their spell slots as dual-prepared spells, at the cost of having one less spell per day of each level.