Some slight tweaks on the good old "RTD" mod, all initiated on taunts with melee weapons. The taunt effect would be usable every 30 or 60 seconds. All of them would need to be balanced and perhaps changed for flavor, etc, but you get the idea.
For scout: The Big Slugger. Taunting with bat gives 5 seconds of guaranteed crits and extra health, and then a 10-second "freeze" on the scout after it runs out.
For soldier: Sergeant Slaughter. Taunting with shovel toggles a one-minute duration "timebomb" on soldier that ends early on his next shovel swing. When he explodes, he does damage equal to two crit rockets.
For pyro: The Ax of Glory. Taunting with ax grants 10 seconds of instant kills, after which time he dies.
For demo: The Homebrew Bottle. Taunting with bottle simultaneously toggles a low-radius 5-second "toxic" effect, "blind" for 10 seconds, and "drugged" for 20 seconds.
For heavy: The Spicy Sandvich. Taunting with sandvich grants 10 seconds of increased movement speed, reduced gravity and then has "burn" toggled for 5 seconds.
For engineer: The Nuclear Wrench. Taunting with wrench gives him a 4x attack speed with it for 10 seconds, then gets "drug," "burn," and "beacon" for 10 seconds.
For medic: The Vivisection Saw. Taunting with saw grants "all crits" for 10 seconds but reduces his health substantially.
For the sniper: The Blade of Clarity. Taunting with his kukri gives the sniper 5 seconds of instant kills and 20 seconds of beacon.
For the spy: The Whisper Blade. Taunting with his knife simultaneously grants him "noclip" for 5 seconds, "invisibility" for 15 seconds, and "beacon" for 30 seconds.