Ubercharge grants neaby allies health and ammo for 3 seconds
+20% Healing and ammo regen per Uber tier
-10% Ubercharge rate per Uber tier
-25% Health and ammo from packs for you and your patient
0% Max Overheal

Uber meter has 3 tiers/charges that builds up overtime, (similar to Vaccinator) as if it's a Dispenser being "upgraded" to the different levels. Each tier slightly increases the healing rate and ammo replenishment, but also slightly slows the Uber rate. Just like a Dispenser, it can't Overheal your patient, and the Ubercharge that gives health and ammo to your teammates is at Dispenser range. Tier 3's Ubercharge has a slightly lowered rate of a level 2 Dispenser, while the tier 2 Ubercharge has a slightly lowered rate of a level 1 Dispenser. Tier one has the same heal/ammo replenish as the small packs.

(I hope my idea(s) make sense and (somewhat) decently balanced but probably needs to be tweaked. I think that the concept of a Medigun having a uber that acts like a dispenser could work, but obviously needs to be lessened to not make normal dispensers obsolete).