It would be really exiting for me if this little guy (or someone else entirely) filled the same sort of ambush/support role Pyro does. With that in mind, here is a set of ability's I've thought about for him. Note: Chemicals are just a place holder, it could be fire or what ever.

Mouse 1: Standard short ranged Chemical spray attack (essentially Pyro's M1 with flame thrower, A particle attack that deals some damage on hit and then apply's a status effect that deals damage over time).

Mouse 2: Either an Airblast OR a small shield that pops up for a split second and deflects projectiles (What is the difference you ask? The latter could be implemented if the developers didn't want the hero to have the ability to push back other Hero's in a way that would be quite intuitive to look at, A shield that quickly forms and vanishes in front of you doesn't look like it should push people back like an air blast does. They could also make it so air blasting doesn't stop movement like it does in TF2, essentially stunning the target mid air).

Q: Fire a Gas canister from your weapon that deals higher damage if the target already has your classes DoT status effect applied (Flair gun from Tf2) (What really excites me about this is that there would be no weapon switching so that removes the requirement for air blasting enemy to puff and sting but still makes it a little harder (if you remove the ability to air blast enemy like in tf2)

E (The Ult): Not too sure on this one, maybe it drastically increases the DoT of your status effect, that could be interesting.

Shift: You can't shoot, you take more damage, but you move faster. (Being an ambush class, some sort of movement buff is required).