I like the idea to allow HMs building living place - "improved" castle.

Also, in addition, to do not allow HMs get bored, I would suggest:

1. HMs may be allowed to train some unique spells/skills related to their guild. Make few to choice from, but you can train only 1 from the list.

2. Those unique spells/skills may be allowed by joining HM-guild or open Prestige class(es) for each guild. You choice new class for your char and train new abilities.

It could be cool to have "ALIGNMENT" for HMs. So, GOOD and EVIL chars could only pick allowed for their alignment classes, skills, spells. That should add some RPing in the game as well.

Regards,
-Guinness