Funny enough, what you suggested is already in the Patreon version of the game. Once you join the quest guild and unlock the gold badge, you get access to a room where each character can get passive skills called "buffs". Each 10 levels a char can pick a new one out of a set of three buffs (aka at level 10, 20, 30 etc), and you can change them at any time. There are buffs that boost your damage, yes, but also ones that half your enemy encounters in dungeons, since Speedu saw that people are struggling with that.
