Hmmm.
Maybe a Metal ammo could be added to a player after picking up a sentry item (maybe Bushido's or custom one), dispenser or teleporter. The Metal ammo level could appear as a little box in the lower right corner with the amount of Metal you have. The Metal could regenerate at 2 metal per second and be used to make sentries, dispenser or teleporter; 50 metal for a dispenser, 75 for a teleporter and 100 for a sentry. Destroying a creation made with metal will give you 10 metal to speed up the metal regen process.
good ideas amirite