Unlike, say, the the Skyrim Workshop, the TF2 mods from the Steam Workshop don't drop into your backpack: Valve's multiplayer shooter needs the Team Fortress 2 team to filter out the best of the best and work them into the game, often abandoning any stats the modder has come up with and just using the skin. So we can do it too. With TF2 now accepting sci-fi weaponry, medieval head-loppers, and boom boxes, the community have filled the Workshop with mad inventions that they hope will send Engies into the Skybox. Here are my favourites.
The High Society
An umbrella that lets the Spy float gently to the ground after dropping from a height. While it's a decent idea, the useage is limited. Here's how I'd fix it: I'd allow the Spy to used it as a hook as well as an umbrella, so he can travel up and along with it as well as down. But I'd also make it his invisibility cloak: he can only cloak while travelling with it. It'd give an interesting range of movement and tactics, and the balance of only being able to cloak while moving with the device would neatly cap off its effectiveness.
The Bird's-Eye Bomber
A new Soldier Rocket Launcher that takes the trouble out of Rocket-Jumping: It allows you to fire the gun down while retaining your aim running forwards, so you can see where you're flying to. The main tweak I'd add would be to make the gun the sole weapon you can use while rocket-jumping: no switching to melee weapons or the shotgun if you've ran out of ammo on the way to your target, and it should add a +25 damage hit to negate the fact that rocket-jumping just got a hell of a lot easier.
Up is down, black is white, and the Pyro is freezing people instead of flaming them to death. The usual reaction on seeing a Pyro is to run, but with this freeze-blaster it would be impossible. Here's how I'd want it to work: main fire hurts and slows enemies down; not as brutal as the flamethrower, and if they shake their mouse they can escape. Alt-fire needs to be up-close, as it freezes the player solid, with a melee attack that cracks them into little pieces. Obviously that second one needs to be negated: an enemy Pyro can melt the ice, and the ice-blast needs to be charged up.
The Jetpack Retrofit
There is a subset of TF2 engies that can use their Sentries to send themselves into the air: this jetpack would negate that particular skill. Balancing against jetpacks is an interesting challenge to consider, so here's how I'd make it work: have the jetpack be a building that the engi has to make before he flis with it. It should run on metal, so it's limited. It's also his sentry, so he has to convert it before setting his gun, which means he can't fly around when it's in that form. This also means it should be sappable: seeing him spin out of control in the air would be priceless.
According to the description, "it werfs notens". A wearable gramaphone for the medic that slots into the new Sound Device hole. It's a beautiful model, and pondering the Medic's place on the team it could inspire (uber) those within earshot. Similar to the Soldier's Battalion's Backup it could allow how a localised boost of everyone around the Medic, allowing them to break a big defence on the offensive, while a smart Medic could give a defense an extra boost in the face of an assault.