As far as engineer stuff goes, I would like to see the following:
base upgrade - New sentry added that you can change in the loadout menu. It would be an assault sentry that builds fast but sits somewhere around level 2 sentries on offensive power, and can't be upgraded. It should look way different from the regular sentry, and sound different, so I kind of think it should fire a goofy 50s sci-fi style laser, or something non-bullet/rocket related. Maybe a flamethrower, as that wouldn't fuck up spies unless they approach it un-cloaked or disguised as their own team, but then it's going to be easily countered by out-ranging it.
unlocks:
nailgun - Replaces shotgun, works kind of like the syringe gun, and whoever you kill drops more metal that only you can get by running over their corpse.
super-slick spray - Replaces pistol, can be sprayed on all your buildings to make a sapper fall off and detonate harmlessly. This causes the spy to have to wait a few seconds before his sapper can be used again, and only stops the sapper once. Can also be used like jarate to create slick spots. Downside is this thing is flammable so your buildings will catch fire/slick spots can catch fire from either explosives or flamethrower/flare gun (enemy only).
welding torch - Replaces wrench/pistol (not really sure which it should replace), used to repair/upgrade buildings and destroy sappers, it can also give them a temporary health boost like being overhealed, and will set disguised spies on fire. Downsides are conflicting with the spray, doesn't repair as fast as the wrench or in big chunks, so it might use less metal in one hit but it won't protect a building. Overhealing would only be about 25% extra health for buildings instead of 50% bonus health like medics get.
I don't know, maybe some of these would be overpowered but I would kill for at least 1) a new assault sentry that you don't need to hide behind and whack with the wrench, to support playing a mobile engineer, and 2) a nailgun or some other kind of semi-auto weapon to replace the shotgun, even if it's weak. The main thing with the welding torch is they should get some kind of anti-spy upgrade (even if I think in a lot of cases, well-fortified engineers are nearly unassailable by spies anyway) and it would let someone else easily spycheck, and it would also maybe provide a little easier to sustain repairing, without repairing huge chunks of health at once. Honestly I wouldn't mind it replacing the wrench, so engineers have to choose between spray/wrench or pistol/torch with their loadout.