Mabey insted of flight it gives you a small base health boost, no fall damage, and a slower fall speed or higher jump hight. inexchange for 20%-40% increase in fire damage taken.

Double jump maybe

or maybe 75 less health on user

If it replaced your pistol, you're already getting the debuff of having no secondary, if it was just a jump boost, or very small 'jetpack boost' (Think something like Red Faction Guerilla before you get any upgrades, mostly used to boost your jumps) Then it would be fine without debuffs, you should also consider that if done like Red faction, it would have a charge too, so it cant be spammed.

or you burn yourself

Haha, "Get set on fire on use"

How about make it simply and aesthetic thing where it just makes him float just above the ground but doesn't actually change how he plays in anyway

I'd say just giving him a small boost above ground when jumping (higher than regular jumps) would work here.

Maybe add a fuel bar that you refill with ammo boxes?

You can only fly for about 5 seconds then go to ammo boxes to get more fuel

Thee could just be a cooldown for the 5 seconds actually and not make it high enough to get much higher than a crooch jump. The problem with using ammo boxes is that if you were playing on maps where it was hard to find ammo it would make it difficult to use agian. Make it have to "reload" cow mangler style and you have solved that issue.

does that make u double jump and don`t receive fall damage, would be nice, but fly it's kinda of bad for the opponents depending on the class

YES FLY

higher jumps,doublejump and falling slower thats what it could have

Or, it can just do nothing becuase its a cosmetic.