Back on topic...
If we DID get a soldier update, what is there really to improve?
I guess clip size for rocket launcher, and maybe a longer range shotgun, but those are minor things.
Same thing for demoman. He is quite perfect the way he is, even though he could use a straight shooting bullet weapon to destroy stickies and to combat really long range sentries.
As for engineer, there's a thread in the TF2 forums about the beta improvements.
Amongst other things, it is said that the engineer will get a new wrench that builds fast but upgrades slower. I see the fun in that; build lvl 1 sentries in unexpected places, and go battle engi to make the kills quicker. I've done it before (not too successfully), but it was fun.