In order to further specialize each class, I think it’d be a good idea for attributes not to be all readily available behind a silver payment, but for each player to have a limited amount of upgrades available for each rank. This way the player has an additional choice between the passives related to his class.
I know this adds extra work such as balancing those attributes, but the game could always benefit from extra specialization.
EDIT:
Attributes are passives for your skills that you can buy at a certain NPC for a certain amount of silver and take time to learn. http://toswiki.treeofsaviorgame.com/attribute/introduction
