Special feat for prepared spell casters: Spontaneous Spell

I want feedback on if this concept is balanced and, if not, how to balance it.

Feat: Spontaneous Spell

After a long rest, while you are selecting your prepared spells, you may choose to prepare one less spell than your allowed maximum. Before your next long rest, you may now cast one unprepared spell with the appropriate spell slot, after which, it becomes prepared. You cannot cast spells this way that are 6th level or higher and the spell must be in your classes spell list.