i had a few small ideas that are most likely stuff and will be shot down immediately but what the hell i'll pitch 'em anyways. goddamn browser is lagging as I type these
ability tokens
basically tokens you can buy in the shop to grant you an ability for the round, such as reduced self damage (for frequent rocket-jumpers) or a very slow health regen when you aren't moving (useful for newer heavies who tank a lot of damage and have no medic present). optionally a small token model could be mounted to the player's torso.
boss skins
an aesthetic feature for bosses which allows the boss to change the way he looks a little bit --would probably just change the weapon models and/or accessories of the boss's avatar, although more complex skins could change the particles, projectile models, or even overhaul the boss's entire avatar. would cost maybe 5-12 boss coins per skin, depending on the complexity of it, and perhaps require that you have played whichever original boss you want to buy a skin for at least a certain number of times. i stole the idea from league of legends :)
custom auras for donators
speaks for itself -- more auras for the donator to choose from. instead of just fire, players could choose from a number of particles to sport such as electricity, or cold fog. would be nice for any donator that wanted to have a theme to their look.