From the description and the looks, this seems to me like a very defensive weapon, so here's my idea:
On Ubercharge:
-All enemies within a medium range of the medic will begin to lose health at 3/4 the speed that a regular medic beam would heal them.
Pros:
-Ubercharges build 30% more quickly than regular medic guns.
Cons:
-No overheal.
-25% slower heal rate.

This will force all imcoming attackers to back off the medic and fire at a distance, making weapons much less effective. Could also be used in offensive to kill injured players within short range of the medic. Since it can't miss, all enemy players in the surrounding area would be killed if at low health.

So, these are my stat ideas:

Pros:
+20% longer healing beam
+15% faster Ubercharge rate

Cons:
+30% faster overheal decay
-40% overheal build rate
Can only Uber 1 teammate with a single Ubercharge

Ubercharge:
Ubercharge provides a large healing sphere with a radius of 450 Hammer units. Every teammate who is in this sphere (including the Medic and his patient if he has one) is slowly healed (48 HP/second)
Ubercharge provides the Medic and his patient 65% bonus maximum health. During this period both of them can't be overhealed. After the Uber ends or if the Medic dies, the bonus health becomes an overheal which begins to decay quickly (just like a normal overheal from this medigun; decays 30% faster than the stock medigun)
While under the effects of the Ubercharge, 30% of every damage dealt to the Medic and/or his patient is returned back to the enemy who dealt it

I thought it would be best to take the name, "The Pain Shield" (Der Schmerzschild translated to English) literally.

Pros:
+25% higher overheal
+35% longer Ubercharge
+25% slower overheal decay rate

Cons:
-25% slower Ubercharge gain rate
-25% overheal gain rate
-10% heal rate

On Ubercharge:
+30% higher heal rate
+25% higher medic regeneration rate
+10% damage reduction
A smaller version of the shield from MvM is projected in front of the medic. Projectiles bounce off the shield and bullets are destroyed. 3 smaller shields circle around the head, torso, and legs of the Ubercharge victim and revolve around them at a moderate speed. In addition, all team members within a small radius around the medic regenerate slowly, can gain 20% of a normal overheal, and gain a 10% damage reduction.

I feel like these stats would give the medic the ability to keep his team safe for a longer time. By putting in a little extra time, the medic could give his team mates a more significant health advantage than one given by another medic. Not only would those be healed have a higher chance of overpowering enemies, they could also gain a similar advantage during an Ubercharge without getting directly healed. The advantage given by the weapon could give a team the ability to overpower an enemy even while losing.

i like mr.$wag's idea

I would assume that this would be a Medigun more suited to defense (I mean heck, even the name translated is "The Pain Shield"), so I would believe that Magicmaan775's idea makes a lot of sense. However, I would like to suggest an alternative to the ubercharge to make it less of the Vaccinator's/Quick Fix's ubercharges.

The +25% Medic regeneration rate I believe should be changed to 33%, so that it would make your life regen +4 hp/s instead of +3.75hp/s, just to make things round off nicely. Now, instead of giving a +30% heal rate, I believe we should put more emphasis on the "Shield" part rather than a straight up healing boost like the Quick Fix;s uber. What about a +10% heal rate (+26.4 hp/s in combat, +79.2hp/s out of combat), but maximum overheal is increased by 10% and overheal build rate is increased by 15% during the effects of the Ubercharge applied directly to a target (which includes yourself)? I also like the area of defensive effect idea Mr. $wag brought up, which is why the ubercharge stats I have now seem a bit... lackluster, to say the least, which is why I add that any teammates in a 450 hammer unit vicinity (a healing bubble that can be seen) have a less powerful version of the direct ubercharge's abilities, such as maximum overheal being increased by only 5% and overheal build rate is increased by only 10%.

Now, no matter if you're ubering someone directly or not, this "bubble" of ubercharge acts like a Quick Fix's, but only applying to overheal. Anyone in the bubble (including yourself, with the exception of disguised spies, unless you directly uber them.) gains a passive +5 hp/s. Pretty lackluster, right? Well, the reward for keeping you and your team at peak health is that once a teammate's health is full and in your bubble, they gain overheal at a rate of +15 hp/s. This will allow the bubble to be more effective defensively, as the +5 hp/s will most likely not save someone in a fight, but say if you pop it and you effectively heal your teammates who are in one spot, holding off an area, the overheal will most likely be more effective.

Without going too much in detail, the bubble that pops up in a slightly more transparent version of the dome similar to when you're placing a sentry, visible by the enemy team so that it won't be unbalanced.

Now, before you say that this will be very hard to fight against and unbalanced, let me start off by saying that this in no way makes you invincible. Even in your ubercharge's effects (where you have a maximum of 240 health), you can get insta-killed with a backstab, one shot by a half-charged Sniper shot to the head, and one/two/three shotted multiple critical weapons, making this a Medigun weak to the Kritzkreig, if you don't dodge properly.) And due to the stats suggested, the uber charge builds at a normal rate (40 seconds continually healing damaged teammates, 80 continuously healing non-damaged ones.) and lasts for 12 seconds, making it weak to prolonged pressure or a well timed Kritzkreig. Not even putting the ubercharge into perspective, the health giving rate is lower aswell. I could go on and on, but I believe I've spent long enough. xD

To sum it all up in a compact list:

Pros
+10% higher overheal and overheal build rate
+25% longer Ubercharge

Cons
- 25% slower Ubercharge gain rate (50 seconds continuously healing damaged teammates, 100 seconds if continuously healing non-damaged ones)
- 10% health giving rate

I don't believe I have enough cons, so if you have any suggestions, pros or cons, let me know, but a new medigun would be cool either way, since the Vaccinator was nerfed into near oblivion.

My stats

+ 25% health
- user stays with 10% speed penalty

On Uber, 75% damage resistance (to all damage types)
-25% uber duration
+25% overheal
-5% speed penalty when weapon is active
+50% uber rate

–10% move speed on wearer
+25% max overheal
+5% move speed on wearer per 10% of Ubercharge, up to 120% max move speed on wearer
–10% Ubercharge rate

On Uber:
Deflects all incoming projectiles (e.g. huntsman arrows, rockets, stickies, grenades, etc.)
+30% bullet resistance
+20% increase in knockback from airblasts
+25% melee damage taken
–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
This is like a Baby Face's Blaster for the Medic, but using Ubercharge percentage to move faster. Would go well with The Overdose, making the Medic play like a beta scout. Had to put some sort of negative side-effect to the amazing Uber, so skilled Pyros would be able to deal with it. Killing this Medic Uber would then be easier with the skill level of the enemy team's Pyros. Conversely, the effectiveness in the deployment of the Medic's Ubercharge also depends on the skill level of the Medic. Good Pyros will be able to seperate the Ubered pair and good Medics will deploy the Ubercharge in a situation where only bullets and projectiles are involved, especially in medium-range battles.

Why not have it get a completely new uber concept, like the other mediguns have.
Here is an example idea:

Ubercharge builds 25% faster.
Upon ubercharging, the medic gains a big (1/3 of healing beam length) shield sphere that absorbs projectiles, hitscan and fire as if it is a solid object. Enemies inside the sphere can only cause melee damage.
This means that the medic can walk forward and block incoming damage from getting to his team as they shoot from behind/from inside the sphere.

I was thinking of that :It would be like the MvM shield uber, but a problem is that it may not be as balanced. Thus, I tried pairing it up with some useful cons, that make it more effective with skill.

Originally posted by {-insert name here-}:

I was thinking of that :It would be like the MvM shield uber, but a problem is that it may not be as balanced. Thus, I tried pairing it up with some useful cons, that make it more effective with skill.

You ended up with too many stats. Less stat weapons are generaly better. (valve wise). They don't add weapons with more than 5-6 stats.

Many people who post stats, don't think of how skill level affects gameplay. If everything was so easy, it wouldn't be as fun, as compared to a set of pros and cons that help encourage creativity in gameplay and learning how to adapt to different loadouts and weapon stats. For example, your suggestion, mishael1. If that was put into the game, the Medic would be like an object, used by the team. Not to mention, that would be extremely unbalanced because no one can get at the Medic to negate the Uber. Something without a counter-balance is not very fair, is it? TF2 is also about learning how to play AS A TEAM. If your team mates treat you like a human shield, how would you feel? Other mediguns have the Uber work BOTH WAYS. The Medic helps the team mate and the team mate protects the Medic. With your suggestion, however, nobody needs protection and the Medic is the only one feeling like he/she has achieved anything.

Originally posted by {-insert name here-}:

Many people who post stats, don't think of how skill level affects gameplay. If everything was so easy, it wouldn't be as fun, as compared to a set of pros and cons that help encourage creativity in gameplay and learning how to adapt to different loadouts and weapon stats. For example, your suggestion, mishael1. If that was put into the game, the Medic would be like an object, used by the team. Not to mention, that would be extremely unbalanced because no one can get at the Medic to negate the Uber. Something without a counter-balance is not very fair, is it? TF2 is also about learning how to play AS A TEAM. If your team mates treat you like a human shield, how would you feel? Other mediguns have the Uber work BOTH WAYS. The Medic helps the team mate and the team mate protects the Medic. With your suggestion, however, nobody needs protection and the Medic is the only one feeling like he/she has achieved anything.

...If a friendly medic popped a shield uber and blocked the sentry while healing me so I can destroy it I would be grateful and feel like working as a team. If I popped that uber and my team would protect me from melee harasment I would feel like working as a team.

Please explain why my suggestion is not balanced skill wise too.

There are no real cons:
1. Uber helps in any situation, exception being faced with a spy team on the enemy side.
2. If it blocks all incoming damage except melee damage, then it functions similarly to the normal Uber, but with an AOE style instead.
3. The Uber would basically negate the effectiveness of sentry nests when it is active.
4. Unlike the normal Uber, it prevents knockback taken from damage! Which is also why 4 is true for this, but not normal Ubers.
5. If only melee damage, at close range, with an already small dps value, would also have to deal with your teammates preventing them from getting in close and personal, would be basically useless in most situations.

Like you said, if your teammates protect you from melee damage, then you basically have nothing to worry about! Pyros can't even use airblast inside the shield so there is no effective way of negating it, except with Spies! And by "skill" I don't mean forcing the enemies to change their tactics completely. It's about adapting to the new situations, with your current loadout and style of playing. It's the difference between asking a Sydney Sleeper Sniper to work with a Bazaar Bargain and asking a Sydney Sleeper Sniper to try using the Sydney Sleeper with the Jarate and Bushwacka.