Ideas and suggestions.

The Mentor allows you to remove skills for a fee, and even in Heroes 5, where there are no useless skills, it was still loved by everyone. In Heroes 3, it would help those who were simply unlucky with the skills they received upon leveling up.

Added 2 minutes later
After removing (forgetting) skills, the hero receives new ones in exchange, just like during normal leveling. It's a complex feature, but I think it would be worthwhile. In the fifth installment, the Mentor was one of the rare buildings.