I thought the original Hot Air Balloon from AW (Fly while firing, 100% more ammo consumed) was quite balanced, as the ammo drain was quite reasonable, and the flight ability was quite balanced as it didn't make players reach RIDICULOUS heights. Its a shame we got a different version.
If I had to rebalance it, it would be:
Fly while firing in the air
(-) +100% ammo consumed while firing
(-) 50% increase in airblast cost

引用自 just this guy, you know?：

Patience (spy watch)
+ infinite cloak
- while cloaked, you cannot move or attack

So basically, the Cloak and Dagger XD

Kinda, but as long as the option is there, you might as well use it for some weapons :)

Actually they can work, you just need to set the baseclass of the weapon to a different weapon, like the parachute or invisiwatch.
Here is an example of the gunboats here:

"Gunboats"
{
"classes"
{
"demoman" "0"
}
"baseclass" "parachute"
"baseindex" "133"
"description" "-60% damage taken from sticky jumps"
"attributes"
{
"rocket jump damage reduction"
{
"plugin" "tf2attributes"
"value" "0.4"
}
}
}

They do work, but the weapon model is not visible (probably because they are boots) but it is visible with backpacks and other models.
I'm not so sure about custom weapon models though.

The Building Betrayer
Spy Sapper

Model: A garden gnome. Evil bastards.

(+) Sapped Sentries and Dispencers will become friendly
(-) Cannot sap teleporters
(-) Is vunerable to damage from all melee sources

The Monado (really?)
Scout Melee

Use Attack2 to select an attribute to temporarily boost your jump, speed, resistance, damage or knockback!
(each attribute lasts for 16 seconds)

JUMP: 50% higher jump height, 15% more damage taken
SPEED: 20% faster movement speed, 25% lower jump height, 15% damage penalty
SHIELD: 50% less damage taken, 35% slower movement speed, 20% damage penalty
BUSTER: 30% damage bonus, no double jump, 15% more damage taken
SMASH: All attacks knockbacks the enemy, 30% damage penalty
All boosts can be applied to all weapons the scout is using.

25% faster weapon switch

50% damage penalty
40% slower fire rate

So... Yeah...
This weapon would honeslty be very hard to make... But if it was, I would really feel it.
( ͡° ͜ʖ ͡°)

These are actually weapons from my server...

The Eternal Avenger
Level X Demo Melee

Dealing damage with this weapon will charge your Avenging meter
When you reach 100%, taunt to take 60% less damage and knockback for 16 seconds
-25% damage taken from ranged sources while active

+70% damage taken from melee sources while active
10% slower movement speed on wearer
No random critical hits
_________________________________________
The Wazzock's Wrath
Level X Soldier Secondary Backpack

Upon reaching 25 health or less, you become a Berserker for 10 seconds!
+25 max health on wearer

+100% self blast damage taken
_________________________________________
The Texas Assistant
Level X Engineer Shotgun

35% damage bonus while metal is available

Drains 5 metal per sec while active
Per shot: -20 metal
33% clip size penalty
_________________________________________
The Lil' Mate
Level X Sniper SMG

Gains a additional fire rate bonus as health decreases
60% clip size bonus
(hidden) 80 ammo instead of 75

25% slower fire rate
Bullet spread increases as health decreases