Valve decides the final verdict anyway, but how does this sound?
+25% faster Ubercharge rate but faster overheal decay?
The Ubercharge could produce a similar shield to the one used in MvM, but could make the medic have slower moment speed, so damage done by the shield will not be over exploited to rush down enemy players and instead be used to protect objectives like control points and payload carts!
Give me some feedback and ideas! :D