New Spell: Sharpen Mind (D&D 5e)

Level 1 Divination (Bard, Sorcerer, Warlock, Wizard)
Casting Time: 1 reaction
Range: Self
Components: V, S
Duration: Instantaneous
Various casters describe this spell in different ways. You might experience a sudden strong memory of where you saw some desired information previously, find your memory palace snap into supernatural focus, or suddenly feel connected to a deep well of memory beyond your understanding. There may actually be several versions of this basic invocation.
When you fail a Intelligence skill check you may immediately cast this spell as a reaction and reroll the check. All bonuses and penalties which applied to the first roll still apply, but not advantage or disadvantage.
At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, after this spell effect is complete, you regain a first level spell slot if you have expended any.

~~*~~

I've come to realize how many of the divination spells were either extraordinarily limited in usefulness by their rules or requirements or were, really, spells for other specialties that just happened to have fallen into Divination by tradition. In the first instance, we have Augury, with a 25 gp cost to cast and cumulative 25% failure chance per daily use. The fact it's a ritual simply makes it not terribly worth memorizing. The second is True Strike, which really only comes into its own when coupled with a higher level damage spell that needs an attack roll. Used with the weapons or cantrips a wizard has access to, there's virtually no point they aren't better off just making an additional attack on the round they'd use to cast this cantrip.

Because Diviners are so centered on the concept of knowledge, the idea of a "mental True Strike" appealed to me immensely. However, the obvious problem with a cantrip of this sort is that a large number, if not a clear majority, of Intelligence checks are made outside of combat, meaning that the main control on cantrips (timing) would be meaningless. As a leveled spell, it threatens to runs directly into the 2nd level Transmutation Fox's Cunning version of Enhance Ability. I avoid the Fox's Cunning because I don't, really, want an ongoing bonus to intelligence, just a one-off boost, which is clearly a reduction in power. With Fox's Cunning showing that my desired effect (a one off boost) is almost certainly a first level spell I built a model where it would be useful. At first it was a preparatory action; before the check was made you used it to get advantage. In this way it was a direct line between True Strike and Enhance Ability. However, a moment's consideration showed that this was clearly too weak as it was substantially weaker than intended and unlikely to ever be chosen. Aside from being extraordinarily situational, it required predicting that a single check was worth a spell slot in advance. By making it a reaction the majority of this concern was solved. Unlike a Cantrip it still burns resources, but it doesn't feel like an extra gamble.

The At Higher Levels for this spell is just a fun sting. It still felt a little weak, and certainly would be a waste to use a higher level spell slot, so my first impulse was to "give back change." The simplest way to do this is;
At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, after this spell effect is complete, you regain an expended spell slot of a level lower than the slot used.

That wording should look pretty familiar. Suddenly, the in-use function of the spell would be for Diviners to regain a pair of 8th level spell slots, which is not the intended effect and pretty wildly overpowered. Lesson being, watch for stray rules that will create unintended consequences. The final version still gives back change, but is only really efficient at it for diviner specialists, which is really the target audience. For diviner's its actually funny, because it's totally spell slot value neutral once you can cast 4th level spells, but at no point are you suddenly *gaining* resources through this spell.