LOL at Level 4 Sentry!
I posted this list elsewhere, but my friends and I had discussed Engie upgrades and came up with this list:
Wrench -> SPANNER or IMPACT WRENCH
Same effects:
-50% crits
-25% damage
+33% Repair/Build Time
SPECIAL EFFECT: Building Armor:
An upgrade for Dispensers, Teleporters, and Level 3 Sentries. Adds 100 HP "overcharge" to the building's health. Does not protect against sappers (I.E., sappers work against the original HP, all other weapons work for the overcharged HP)
As a bonus, Armored equipment does splash damage in ratio to the amount of armor on the equipment when it blows up (either from damage or self destruct)...
...downside is that the equipment slews/dispenses/teleports 20% slower.
So it's good for forward equipment. Armor costs 200 metal for Sentries and Dispensers, 100 for teleporters.
Only the SPANNER/IMPACT WRENCH can repair armor; regular wrenches only repair regular HP.
Building armor is incrimental; I.E., the first whack gives 15HP overcharge, the second gives 15 more, etc, etc, until fully built.
This encourages Engies with spanners to teamplay even more than usual.
Pistol -> TAZER or RAILGUN
RAILGUN is from TFC (or TFF... I get 'em mixed up, with the small amount of time I've played) You can charge them up for more damage; railgun shots ricochet depending on how long they've been charged.
The TAZER has 15 cartridges; if they hit the opposing player they're "paralyzed" for 3 seconds. Does no damage otherwise. Has half the range of the flare gun. Spies who are hit are paralyzed, but it does not drop their cloak or de-costume them.
Final caviat; the cartridge must charge 10 seconds. Cartridges charge the moment they're loaded, even if the weapon isn't drawn. An early fired cartridge has no effect.
Rifle -> AUTOSHOTTY
10 cartridges for full load. Half the ammo carried, no "pulling the reload slide" between shots. Play left for dead, get an autoshotty. You know what I'm talking about.
-10% damage per pellet, wider pellet dispersal. Some of the cartridge's fill and explosive is used to operate the gun, causing less impact on target.
PDA UPGRADE -> Add THE BARRICADE
I posted this elsewhere, but THE BARRICADE is Model D, the narrow one with the humorously blinking light.
THE BARRICADE has 100HP, is as tall and wide as a dispenser, and is transparent to Sentry Guns. Has a 75% to-hit for the other team, meaning if you fire at or through it, 75% of the damage is absorbed by the barricade, 25% is transmitted to whatever would have been hit behind it.
It cannot be sapped (what are you going to sap, the light bulb?), but before you hate me Mr. Zaw, both the Spy* and the Pyro can light it on fire. It burns at the same rate as people. Costs 150 metal to build.
(* I imagine teammates seeing the spy take his cigarette out and putting it against the barricade.)