For passives, that's what conversation bonuses are IMO
Sustained skills would be interesting but since there is no mana in this game, I'm guessing the cost is AP per turn? There are a lot of status type skills with set turn lengths tho so I'm not sure how different they would be from sustained
As for skill revamp I would like a separate shield skill tree from 2 hand melee.
The skills books are ok to me, cleaner and more versatile than skill trees
Your skill damage already scales with it's affiliated stat, more mastery in the skill talent lowers your AP use. I guess your skills doesn't scale with your char level but I feel that's more a personal pref, I don't see a need for it.
Things I want to see in future additions are martial arts skill branch for melee with either fists or maybe claws and staffs (bos)
I don't know if you've ever played guild wars but I would really really love a Mesmer type class, with skills that counter other skills and punish the opponent for using skills that you don't want it to
