Have I got the concept for you! Simple and ellagent, yet bold and refined!
I was thinking, the only thing his sappers cant do yet, is turn an engineers sentries against him.

this sapper is better for hit and run tactics to cause momentary chaos amongst engineers, sentry nests, and arivving team mates.

sapper when placced only lasts 3 secconds before falling off on its own, but durring those 3 seccond the sentry focuses fire on its own team.
WARNING! this is for experianced spy's only! if you are disguised as an enemy team mate while the sentry switches its alliance you will be in danger! so drop the disguess as soon as you sap! but make sure you re disguise before time runs out. this is a tricky sapper to use, but can be of great help to your team by taking out the engineer for you!


BUT!!!!!

the sentry takes no damage from your sapper and hitting it during this time period would act like your hitting a frinedly sentry. In other words, when under the effects of this sapper, you and your team cant hurt the sentry, only the original team can

so, when you sap any sentry it then for 3 seconds turns and attacks its own team.

here are the cons though
(-) has a 6 second recharge before placing a 2nd sapper
(-) it only works on sentries level 1-3 and mini sentries.
(+) slightly more reach OR you can under hand toss it on to a device 3 feet away while disguised but still a very close range and need to aim at the sentry. NOTE be carefull though! the engineer could see the item toss from your disguise to the sentry.
(+) during those three seconds engineers will be unable to take the sapper off.

so recap: "the intermisssion" will breifly interupt enemy engineers. How? it can be tossed at a close range onto an enemy sentry to turn it over to your side for 3 seconds where it attacks the nearest enemy target, including dispensers, teleporters, other sentries, and original team members. this sapper does 0 damage to the sentry, and can not be used on dispensers or teleporters. it has a recharge rate of every 6 seconds so as not to spam....to heavily ;)
remember, that if you or your allies try to take it out its just a waste of amo during the sapping period. ALSO! enemy sentries dont boot down then back up again as if sapped with a regular sentry, it just emediatly turns and carries on. so as soon as that sapper goes off, you better be cloaked up or your not getting away, and your team mates better be out of sight!
engineers dont get an alert on their PDA, that their sentry is being sapped! however their will be a little glitching on the PDA when looking at it to show something is up. but an engineer will have to constantly look away from the battle and you if he is going to check for this glitching.