Guessing the next upgrades...

[Elixer]

[Angry_Squirrel]

only one I can think of
is rocket replacement for soldier

Missile Pod
4 ammo, 16 reserve
Slow reload - same as regular
60% damage
If one gets with a character's length of an enemy, it will "home" in on where they are - so if you narrowly miss, it will zoom towards them, but not follow them around. It only changes course once. Does not lock onto sentries.
Right Click - fire all four at once, equivalent to 2.5x rockets hitting at once.

^ that's my idea, but of course the %'s and numbers would have to be tweaked.

[Makaze]

Format ganked from Striped

...so here's my list:

* Engineer:
o Tank Tread Sentry: Mobile doom weapon

* Demoman:
o Timed Dynamite -- You saw the demos, now you get the dynamite.
o Sticky Grenades -- not the winbombs, but like a movment slowing grenade, like steel foam
o 307 Ale -- "Let me tell you how ay lost me eye..."

* Spy:
o Flashbang -- WHo needs cloak when their eyes are sizzling
o Blackjack -- Replaces the k'nife, stuns/slows makes the enemy have aiming problems
o Halucination Grenades - - (Ala Quake era TF) Doood I'm trippin

[Angry_Squirrel]

IIRC there was something about finding a Beartrap in the model viewer, speculation on that being somebody's weapon replacement? lol

Also, those dynamite sticks, I've played with those before. They were... OK. Then again in the context they were given to us, as a part of grenades for all classes (Sniper got no special grenades LOL) they weren't very aimable and had limited ammounts so that hampered them a bit I guess... IMO 8 stickies = more flexibility than dynamite...

[Makaze]

[Tyakoto]

I had an awesome idea for a Sentry upgrade.

-Eliminate Stage 3 Sentry, no rockets and such.
-Make it so it starts at stage 0, looks like a normal sentry but doesn't fire. Upgrading it lets it fire like normal. And upgrading it again puts it to Stage 2 (which is now Stage 3).
-Give it 25% less ammunition
Sounds weak? Here's the kicker.

--Let it adhere to walls or the ceiling when in range. Spin it 3 times to make it spin around. And when you spin it a 4th time, instead of going back to normal, it now is sideways, wanting to stick to a wall, spin it 5-6-7 and it spins around on the wall (leaning left and right makes it adhere to either side). 8th time, and now it wants to go to ceiling....... or you could make it so it tries to adhere to the ceiling or floor on the neutral-3 spin, and walls on 4-7 spins. That'd be quicker truly.
--It's still taking 400 metal to fully upgrade it like a normal sentry, but now it's more of a surprise tactic and a sneaky doorguard if you could get it to adhere to a doorframe (at least the thicker ones) rather than being a point guard. Imagine the wacky places you could put these things. And use as steps even (Dispenser Hop, Wall Sentry jump) the possibilities make Engineer a helluva lot more interesting in that respect.

[Ayeaka]

Only have ideas for the Spy, really.

Revolver>> Tranquilizer
Pros: Slows enemies, quieter than Revolver.
Cons: MUCH weaker than Revolver.

Sapper>>Hacker
Pros: Converts enemy equipment to own team.
Cons: Sentry gun still fires during conversion OR takes longer to finish working. (Not sure which would balance it better.)

Butterfly knife>>Piano Wire
Pros: Can use directly after uncloaking.
Cons: Much slower to use, can't chain-kill like with knife.

[Purplecat]

Demoman

Nuke Launcher