I would like to suggest introducing a secret NPC for base stat changes, available upon the completion of a quest or for a fee, such as 500k coins per stat point. This could potentially serve as a great gold sink for end-game players.
For example, this NPC could be a night hag who has the ability to change the nature of a man, a reference to Planescape: Torment. She would appear in her human form on the surface and in her natural form anywhere else.