I imagine short simple stuff like the stock taunts, like something along the lines of Medic adjusting his glasses while laughing, or Soldier pointing to the ground while saying "down and give me 20", stuff like that.
I could also settle on those being implemented on new weapons (the dynamite pack for example has its own custom animation) if a taunt loadout its still out of the menu.
I think it would work nice to give a little bit more of variety on the cosmetic aspect since hats cant be added without it being too intrusive to the aesthetic.