Advanced and optional: Dangerous knowledge

This non-official homebrew rule builds upon Learning cantrips to suggest that magic-using classes can try to learn and cast spells of a level above their current cap (half their class level, rounded up)
» For magic-using classes, learning a spell above your cap requires a Wis/Int/Cha check plus proficiency to learn it, and a Wis check to avoid mental trauma causing a disability (see the Limitless Heroics expansion).
» For each check, roll one die for each level the spell is above your cap, and choose the worst result.
» Casting a spell above your cap requires another Wis check to avoid mental trauma causing a disability, every time. Roll one die per spell level above your cap.
» Thus, a 4th level wizard who’s expecting to encounter demons can try to learn Banishment, a 4th-level spell, but they’ll roll an Int check with a 2-dice disadvantage to see if they learn it successfully—and whether successful or not, a Wis check with a 2-dice disadvantage. If they learn it, then when they cast it later on, it'll require another Wis check with a 2-dice disadvantage to avoid taking mental trauma.