I don't see why one out of the four unused classes would make it overpowered. Four might, but one not so much. I would make it cost 10 tokens and 30 mil gold to get one level. You would get 1 hp or 1 ma or 1 mv per level (1 hp 1st level, 1 ma 2nd level, 1 mv 3rd level, 1 hp 4th level...etc.) Caps for healbonus, blastdam, undead control, etc would all be capped as a 4th class.
BTW: I didn't lose steam on this subject, I can't code and I wouldn't push such a project unless it was me putting forth all the effort. Besides, I thought it was too much of a longshot so I was going to just drop it. But since it's out here, I'll post my thoughts.
The whole idea of adding a 6th class isn't because people are bored. There's plenty ingame to buy at avatar. The problem I have is we went from a 4 of 4 class system to 5 of 9, and it kinda sucks. It was the one thing I thought ROCKED on Sloth. I really don't think it would overpower characters... I think it might actually balance everyone.
-Tox / Brad