Well right now the engi is pretty much one of the weaker classes in the game, considering that his primary weapon, the sentry, hasn't changed at all, which puts a cap on his combat effectiveness.
As people learn to deal with guns, anybody can take one out, and now even their value as a strong deterrent is likely to be eroded as well.
I somewhat disagree!
I think that the other classes are starting to match the Engineer in effectiveness, not the other way around.
Remember, the Engineer has a Heavy Weapons Guy/Soldier combo robot built with a never-miss aimbot, a dispenser that keeps health and ammo and metal at hand, and a teleporter system that can move his most dangerous people right to your front door.
And then Valve allowed me to destroy stickies with my shotty instead of just pushing them away. Insane!
If it wasn't for Ubercharges, Engineers not asleep at their post would be neigh invincible.
Only the Bonk! beverage has had a significant impact against Engineers, as the sentry is dumb enough to track the first thing it sees... including Scouts dancing in front of them Bonked to the gills, and not that soldier over there spamming rockets at it.
And then Valve let me upgrade dispensers and teleporters to make them faster and better. Even more unfair! Heck, if it wasn't for the goodies given to the spy in those ninja updates as well, I'd hate to play ANY class other than Engineer.
Even the new watches: Dead Ringer and Cloak and Dagger... they're only useful against watchful Engineers in the hands of Expert Spies. I have headphones, and I listen for cloak sounds. I have a pistol, and I spam it at anything that moves. I have a shotty, and I pump a few rounds into anyone who walks toward me.
I'm just NOW starting to feel balanced against the other classes. But when Soldier and Demo get their new toys, I'm sure to start feeling the hurt and wanting new toys as well. For the first time since updates began.
And I don't want new toys that make the Spy's job harder. Dispensers that shock spies and sentries with flamethrowers... I've never liked those ideas. First off, one of the few tactics that actually work against attentive Engineers are spys camped on dispensers, and if you are really paranoid about that possibility, spam some bullets around your dispenser and look for flashes.
And if anything, Sentries need MOAR RANGE, not less. You want a flamethrower sentry? Get a Pyro to stand inside a dispenser. I wouldn't want to trade long range bullets and missiles for a little gout of flame

And if the idea is to replace pyro spy checking with a rotating beacon of flame, I'm even more opposed.
If there's any class in TF2 that brings home the "Team" aspect, it's Engineer. A lone wolf Engineer trying to defend an area building his own sentry and dispenser, who has pissed all the other Engineers off so much that they won't help him, finds the task neigh impossible. I need a minute and a half to finish all my building alone. In that time, I need my team to keep the enemy at bay. Or I need an Engineer buddy to assist me. And when there's a spy about, I'd really love some pyro spam. In all cases, I don't want that team dependence to be dropped. Engineer is so useful BECAUSE he is a credit to team; but a Lone Engineer is a hard class to play.
ONE LAST THING: I want an achievement for turtles. Call it "Slow And Steady." You get it for standing next to your dispenser for five minutes. Even though I hate turtling, it must be done.