With the increased importance of attribute potential max values because herbs don't train them that easily anymore, wouldn't it be time for a Potion of Potential. A potion much like a Potion of Gain Attributes, but obviously it only affects potential maximums.

Behaviour patterns regarding B/U/C status:

Type 1: Very generous
cursed: raise one potential stat by 1
uncursed: raise all potentials by 1
blessed: raise all potentials by 2

Type 2: Probably more realistic
cursed: lower all potentials by 1
uncursed: raise one potential by 1
blessed: raise all potentials by 1

Type 3: nice combination but probably still too generous
cursed: lower all potentials by 1
uncursed: raise all potentials by 1
blessed: raise all potentials by 1

Mind you, AFAIK this item isn't in the game. If it got added in any of the prerelease versions, I guess I look like a proper idiot now huh?