I don't do KitPvP, but... is it that hard to have or find a Kit plugin that just has balanced classes like other PvP games?
We know from the EULA that donators can't get advantages. So maybe they just get some cosmetic things, like in other games. Weren't people talking about letting donators color the enchantment effect on items, that's something? And maybe they could color all armor, even non-leather. And maybe things like vanity pets? Maybe cosmetic kill animations at the location of a players you kill (explosion, tombstone appears, chicken-splosion, skull and cross-bones banner, fireworks, etc). Custom objects for weapons? Back vanity items like wings, backpacks, banners, etc.
Just spouting out random ideas.