For what it's worth, I (as a Heavy) played pocket to a Vaccinator Medic once on Mannworks and it was like I could never die. I think I have still not done better than that w/r/t kills and damage dealt as reported by the "Your Best MvM Moments" loading screen.

I think you'd still want at least one Kritz Medic, though, or else high-health tanks would tend to push the line back quite a bit. Or maybe Vaccinators with Kritz Canteen sharing would do it.

Firing speed vs. penetration upgrades would largely be a decision based on the composition of the upcoming wave: firing speed for giant robots, penetration for large numbers of smaller bots.

I've heard stories of nutty stuff like a single Engineer with five Pyros on the Bigrock missions, one of whom (Pyros) was dedicated entirely to Axtinguishing. We've done expert Decoy with no Demoman (replaced by a Heavy). Also expert Mannworks with a spy to replace both Demo and Scout, though that's not especially unconventional.

There's also a Steam group devoted entirely to the concept of subverting the MvM metagame. Their group name has some adult language in it, or else I'd post a link here. I'll give you the link next time I see you online and let you decide if it's fit for posting.

http://gtm.steamproxy.vip/groups/MVMFUCKERY

We ran a Spy (replaced Demo and Scout), two Heavies, two Engies and a Pyro last night on Coal Town. As with the Spy on Mannworks, this composition is not too extreme in terms of subversion, but far enough away from most people's locus of safety to make things interesting.

I think it's fairly well established that Spy has a place in MvM (even in expert, and especially in Mecha), and despite the common initial reaction to Snipers (namely that some CoD refugee has come to waste everyone's time), I've seen the class used to good effect.

So it seems that the thing to do now is to find an effective play style for Medic in expert missions, and then start messing with the head count of each class.