Idea:
A wearable backpack for the engineer. Pretty much it's a portable dispenser. Similar in function to a medigun, but also replenishes ammo.

Looks like:
Normal dispenser, but replace the side attachments of the dispenser with a fuel pump. When equipped, engie holds the fuel nozzle in front of him. On click, discharges fuel on target.

What's it do:
Gives engineer a mobile health / ammo replenishment for those he is dispensing (similar to a medic). Continuos healing / ammo replenishment at half the rate of a level 3 static dispenser.
(i.e. if a level 3 dispenser would give 60 ammo every 5 seconds / replenish 60 health over 5 seconds, then the replenished person would receive 30 ammo / health over 5 seconds.

Limits:
No overheal. Perhaps give additional ammo when full. Health kits and ammo that the engie runs over helps replenish the dispenser.

Why:
Could give engies an additional role in the game, through mobile supporting of heavies or pyros.

Why share it here:
I think it's a good idea, but I would never be able to make it. I think it could work well in the game, and if you're the guy to make it happen, let's see if it's popular and workable.