My head is exploding of ideas so here they go!

Pro's:
+on medic: when active +10% fire and bullet damage and 15% explosive damage resistance on front, when crouched +20% fire and bullet damage resistance and 30% explosive damage resistance on front (cuz when you are crouched the shield would cover more area since you are with less exposed areas)
+25% ubercharge lenght
+10% ubercharge rate
+2 HP regenerated per second (added with the normal +3 HP regen)

Con's:
-10% movement speed on wearer
-20% overheal building rate
-10% healing rate

Uber:
Deploys a similar shield of MVM, but instead of dealing damage it causes players to slowdown to 20% of their speed while passing trough the shield
Shield also destroy projectiles on contact and reduces 75% of bullet damage
Shield has the same size of MVM shield
Uber (shield) can be turned off without wasting uber and be turned on again with alt-fire (you can change weapons and the uber will still be there),but you need to reach 100% to use this and you can't gain more uber healing people while you uber don't reach 0%, also it takes 0.5 sec to turn the uber of and 2.5 sec to turn the uber on again
While the shield is active you can't change weapons

well that's it, tell if you liked people :3

+100% more stats? sugjestions

25% larger uber bar(takes 25% longer to build but laster 25% longer)
uber reflects attacks back to attacker
fire and melee does not reflect

I think it should be:

Pros: (To Medic only and not the patient)
+%25 bullet resistance when weapon is used (In hand)
+%25 explosive resistance when weapon is used (In hand)
+%15 fire explosive resistance when weapon is used (In hand)
+All listed resistances are increased additional %40 (depending on built Uber.FULL=50-Keep reading, trust me)
+Addtional 5 percent if crouching
+Resistances are increased addtionally another 10 percent if healing a target.
(Total 75 percent if all met-Accept fire resist which would be 65)

+These effects include crits. Meaning 25-bullet resistance = 25 crit resistance of that type.


Cons:

-No overheal.
-Uber gets drained if injured. (Makes sense, since the gun would be using its uber to protect you more) (100 damage = 50 percent of uber drained./The resistances will protect you and the more uber build you have, the more uber will be saved from attacks)
-Can be damaged %25 more from malee attacks.

Uber:
+Resplaces Uber with the large MVM sheild that will slow target trying to pass through, stop ALL protectiles and 75 percent of bullets.

+Along side this,any enemy that passes through it will be slowed and marked for death for 2 seconds)

-Marked for death during uber.

Aka, the main counter to this would be just simple malee.

Stats:
While active,
-25% damage taken from ranged sources,
Heal target recieves -5% damage from ranged sources,
-5 Heal rate
Player moves 20% slower,
Does not get speed bonus from healing faster classes,
Ubercharge rate -10%

On Ubercharge,
Neither the Medic using it nor the heal target can recieve more than 15 damage per instance of damage.

Notes:
My idea for this is basically the reverse of the Quick-Fix. While the Quick-Fix grants much mobility to the Medic, this takes it away. Not to mention that while the Quick-Fix Uber is weak to focus-fire and large enough bursts of damage (think a Demoman's Stickytrap) and is strong against steady streams (think the Heavy's minigun), this one eats up burst damage like it is nothing, but is weak to steady streams of damage.

What about a medi gun that gos by the name of the munitioner and it does

(-)Cant crit heal
(+)Regen ammo at 25% a second (for you and patinet)
Ubercharge-infinte magazine size
+50% weapon switch speed
+25% fireing speed

The Forward March
+ Uber is replaced with a shield, which soaks up bullets/projectiles.
+ +50% uber duration time.
+ Overhealing patients does not effect uber build.
- -35% base charge rate.
- Crits/Mini Crits penatrate and temporarly disable the shield.
- Damage soaked up by the shield takes away uber, depending on damage.
- Enemys can walk through the shield.

The "shield" is based of the MVM shield, but a bit smaller.

+if the healer and patient were damaged by bullets, it charged the uber faster
+uber grants a barrier that redirects projectiles back

Okay, got an insane idea for stats.

Uber Bar +50%
up to 25% damage reduction on self and target and from all sources based of Ubercharge, Damage reduction reaches 25% when Ubercharge bar is at about 25%

Ubercharge Loss on damage equal to damage absorbed by damage reduction.
Ubercharge Lost on Medigun Swap.

And here's the crazy Part.
No Ubercharge Ability.

The stats i just gave means you Kinda have a constant Damage reduction when you are healing a target, therefore the balance i came up with is this medigun actually doesn't have it at all.

+ubercharge grants a protective dome around the Medic
+every kill-assist grants 25hp to Medic and 15% uber charge
-Medic's feet is unprotected and will receive more knockback during ubercharge

My suggested stats on this weapon:

+When healing: User and patient's passive resistances to all damage sources will gradually increase up to 75%
+40% faster heal rate
+40% faster ubercharge rate
-50% reduction in maximum overheal
-/+ Ubercharge will grant nearby allies to heal 80% of damage dealt to the enemy targets
and immunity to all movement-impairing effects and be critboosted