(Weapon adjacents being reworks / redesigns of weapons, i.e. the Uberspritze to the Ubersaw, the Mine Layer to the Scottish Resistance, or the RPG to the Direct Hit)
Some weapons I would love to see versions of added to TF2C:
Force-A-Nature
Sandman (would need a rework)
Buff Banner
Pain Train
Backburner
Natascha
Frontier Justice
Blutsauger
Old Darwin's Danger Shield (+25 max health)
Your Eternal Reward
Cloak & Dagger
Even if they were reworked, buffed, nerfed, or changed visually, I think all of these would be great for TF2C.
one of my ideas once