Arelith

This might be impossible, but the suggestion is to allow a caster's Level X spell slots to be filled with spells up to and including spells of that level, rather than solely of exactly that level.

For example, rather than a Wizard's Bull's Strength being able to be put only in second level spell slots, it can be put in any spell slot of second level and above.

Metamagic is the only mechanical way to do this.