instead of new characters how about expansions on current characters, similar to new clothes , but you could do new equipment slots. Eye patches, rings, hats, all with small but stacking properties that could enhance gameplay. I have always laughed at the 500k gold from a battle, what if you could get a hat that was 10% of unhoarded hahaha that would be funny. or a eye patch that let you do +5% att for 100 hits when activated resets in 24 hours
Buffs work well in other games, it might not be such a bad idea to add some new slots to buff/boost. Maybe craftable stuff, but needing alot of stuff to create?