Butterfly knife replacement: Cannibal's fork. Does no damage from the "front," but heals 25 HP on a backstab. Dignified Mediterranean "om nom nom" sounds optional. Voice clips complimenting the chef also optional.
Sapper replacement: gag kit. Contains a collection of tools for subverting enemy buildings. They can be deployed while disguised, and which one is used will be determined contextually.
Deploying the gag kit any old where drops an Inflatable Decoy. Only one can be deployed at a time. Drop it on the ground and in a few seconds it will blow up to become a "lifelike" facsimile of a Spy. It is immune to bullets, but a single melee attack (or perhaps two) from any class will destroy it. If it is in an enemy sentry's range, the sentry will fire at it continuously but ineffectively (indeed, it will fire past the dummy, hitting whatever's on the other side) until its ammunition is depleted. It is dangerous for an Engineer to take out a dummy by himself, as he can be harmed by his own sentry; better that a teammate should do it.
Use it on a teleporter to deploy a Telebomb. The affected teleporter will turn a highly conspicuous yellow color and change sound. Anyone dumb enough to go through it while it is in this state will explode violently ten seconds after teleporting, damaging nearby allies (and buildings) and dealing self-damage equal to that character's maximum overhealed HP minus one (so if fully overhealed you can survive your own explosion). A person who is going to explode will flash yellow and beep. As with a sapper, one or two wrench hits will return it to a functioning state. You can also enter a sabotaged teleporter yourself in a suicide attack; with the Dead Ringer, it may be possible to barely survive the explosion. Or maybe not.
Use it on a dispenser and said dispenser will undergo some obvious visual change. Standing within the dispenser's range will refill enemies' ammo and metal extremely quickly, almost instantly, but it will drain a proportional amount of health from those enemies. The amount of ammo refilled will be sufficient to refill all weapons, so standing near it with an empty weapon or zero metal will be deadly (possibly, it won't drain HP lower than 1, and will simply do nothing to enemies with 1 HP). It still functions normally for disguised Spies. It can be restored to ordinary behavior with two whacks of the ol' wrench.