I was thinking more towards this:
Deals no damage to building health while sapping
Once sapper timer goes out the building is destroyed in an explosion that damages anyone nearby

My idea
Green text:Destroys building after 3 seconds
has Exsplosive damage
if shot exsplodes quicker
red Text:Less sapper heath
100%- sapper damage
has to recharge to before using it again(10 seconds)

My idea makes alot of sence
:D

Also if you are on fire when you have it out, you immediately take explosive damage and the people nearby you take damage as well.

+Upon destruction, the building explodes and deals damage to nearby players and other buildings
-10% longer building destruction time
-50% sapper health (engineers only need to hit it 1 time instead of the usual 2 to destroy the sapper)

It'd be a lot harder to destroy a building with this but, if you use it against careless engies/engies out of their nest, it has the potential of single-handedly do a lot of damage to said nest.

Cool

hmm maybe it wouldnt do any sapper damage but instead it just sticks on to the building and the moment the engie hits his building with the wrench for upgrading/repairing it activates dealing explosive damage to its surroundings

I think it's about time we got a sapper that out-and-out destroys buildings quicker, but doesn't disable them whilst doing so.

here goes

Upon detonation, all players and buildings in its radius recieves 75 damage
-30% sap duration

Does not disable buildings
-100% sapper health

I agree,

Normal sap time
- No damage while sapping
+ An explosive blast that destroys the building and does splash damage to nearby enemies when timer runs out.

My two cents on the subject:

+ Does not directly alert enemy Engineers
- Makes a loud ticking noise

I agree with Warlord Steel Shroom, Windy, and P♥T Mojoey. One of the things I like about this is it might make the engineer hesitate to save his buildings depending on how close he is to it, and it really encorages him to take care of the sapping before dealing with the spy, thereby encoraging teamwork more than usual and having a team mate deal with the spy while he tends to his buildings. As for sapping time, I am not sure how much longer or shorter it should be to be considered balanced (or if the sapping time is changed at all) or how much sapper health it should have. I think that maybe it should be set off by other allied explosives, at the very least other sappers of the same kind within eachothers range. Also perhaps the buildings will still be functional while sapped with this sapper.

+20% sapper durration
Does not do damage to buildings untill the timer runns out
When the timer runns out it kills all nearby enemy`s and buldings
Everybody can remove this sapper
+100 damage to self

+sapper explodes and destroy any buildings in range
+explosion does 100 damage to enemys in range
-sapper does no damage to building
-explosion can kill you
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in MVM
-sapper doesnt slow down robots
-when you die the sapper will be removed
+sapper got timer
+can upgrade explosion damage and range
+explosion doesnt hurt you

+Creates An Explosive Radius 7 Feet out, and around the plotted building of the area (powerful damage like sticky bombs or sentry busters.)
-Takes Deploy Time 6 seconds to prime, and set(think smart bring friends.)
+Engineer must hit metal points, hits the wrong area it explodes(Treat with Caution in other words(takes 4 hits in a matter of 13 seconds.)
-does not disable buildings(explodes.)
+Can kill you(plot you're escape.)
=You only have A few of them, must go back to the locker to get more-cannot retain on ammo or past guns.
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Robots-same thing.
-one advantage is the Robo Engineer can hit where ever to remove sapper(fairness,but of course still four hits.)
I suggest this is An nest take down so guard it with you're life, if you want the targets down.

I hope these ideas help.