Here's my idea for this weapon. I'll explain my reasoning for the stats below.

The Treadmill
(-)-75% Ammo (50 shots)
(-) -15% Firing Speed
(+/-) Firing speed slowly increases while staying revved up (from -15% to +50%)
(+/-) Movement speed slowly increases while staying revved up (from stock to 35%)
(-) Damage slowly decreases while staying revved up (from +20% to -5%)
(+/-)Each Kill refills ammo for all weapons

Explanation

First; -75% ammunition, resulting in only 50 shots. This is because with each kill, all ammunition is restored. So, as long as you plan accordingly, you should be fine.

Second; -15% Firing speed. This goes with the next 2 stats.

Third/Fourth; Firing/Movement speeds slowly increase while staying revved up. This is to encourage heavies to be a little more like the roamer class, not depending on that dispenser to stay full on ammo.

Fifth; Damage slowly decreases while staying revved up. This kind of goes along with the previous two. It's to promote not sitting down in one place, to not be lazy. But, if you're okay with less damage, you can run around with +35% movement speed and +50% firing speed. Although the increased firing speed with only 50 shots.. You should be careful.

Sixth; Each kill refills ammo for all weapons. This is to make sure the heavy using The Treadmill never runs out of ammo. Skill for more kills.

Aanndd.. that's about it. The whole idea for, well, MY idea on this weapon, is to get the heavy up and moving. Instead of just blatently W+M1nning the enemy team and getting backstabbed around 100 times, the heavy using The Treadmill will start to plan what he will do. With only 50 shots, the heavy must think about his kills.

"Should i waste my ammo on that scout burning with 5 health, or take down that soldier in the air with a marketgardener.."

Please tell me what you think in the comments!