thoughts off the top of my head.
Something that increases spell power as though you were casting one level higher, but you take damage equal to the original spell level.
Every spell you cast costs twice as much.
All fire spells are changed into poison.
Your Charisma goes down 4, and no spell you cast is able to make someone look better, only worse.
Take 1d10 force damage after a long rest, represented by bodily functions going HORRIBLY wrong.
Seeing any of (Insert monster category here, Humanoid, beast, Fey, etc.) Causes the charmed condition, and you cannot damage them.
Same as above, but Frightened instead.
You slowly change into another race, and roll on the Reincarnation table to decide what race that is. Nothing else about you changes.
Lycanthropy.