Perhaps Ubercharge could be the MvM shield, with some tweaking of course. I have some ideas, but have no clue how to make the shield into a standalone medigun. Here's some ideas to mess around with and put together, though these are only for how the ubercharge will function, regardless of stats.

-Blocks and eradicates projectiles still, but only resists hitscan
-Shield is wider and/or taller
-Shield is in a radius around the Medic instead of being a frontal wall
-Remove the actual shield's ability to deal damage
-Reflects projectiles instead of blocking them
-Can be used at anytime like the Hitman's Heatmaker's Focus bar so long as there's any meter