Do the more original weapons first: Syringe Gun, Blutsauger (perhaps adding a "critical hits" thing only to the Syringe Gun), remote bombs, backstabbing knife.
I'd like to attempt a TDM-working "Disguise Kit" item, but I think it'd be overused in a deathmatch ("Now EVERYONE is on the opposite team!") plus you wouldn't be able to distinguish it to members of the team the spy is on compared to others for the mask effect.
A Disguise Kit would be awesome, but you wouldn't be able to disguise peoples' names with the v11 changes... I was thinking that a "TF2 game mode" or server script would hide peoples' names from far away - making the disguise believable from far away and just about so close up. (although being so close as to touch the person gives it away if they're observant, like the game.