You've probably put your finger on the basic engineering/human factors design problem. I suppose part of the problem is defining the mission, and putting the right number of guys into the vehicle. (This story makes it sound like you'd need no fewer than 3.)
If I read these guys' comments correctly, the primary requirements are firepower and armor, with speed being a second-tier requirement.
It also sounds like there's a serious need for close-in, anti-personnel armament ("we were fighting out of the hatches with rifles and pistols...") They also need some heavier stuff (probably not an M-1 sized gun, though) -- I don't know what sorts of "short, big guns" might work. Sounds to me like really big firepower needs could be met by a combined-arms tactical solution, rather than trying to put it all in one vehicle.
There may be some space to be saved in fuel, engine, and systems requirements for an "urban" tank. We certainly don't need a 60mph-over-rough-terrain vehicle. Nor do we need many of the higly-advanced fighting systems (such as the gun-steadying mechanisms). Basically, we need a heavily armored, short-cannoned (fletchette rounds?) fighting vehicle, with moderate speed needs and moderate range. That may shave some space off, a decent amount, but we still would need.....I'd hazard 3 guys, as the "main" gun loader would not be a priority task (if it was, just use a MBT to level the damn town). Feel free to change or add.
For the urban fight, you move at the speed of clearing buildings... since your objective is killing the enemy here, not bypassing him. You can't go 4GW on him, unless you have a way to bypass the tentacles and strike at the head of the octopus -- that is a function of intel (mostly human) and special operations (working together, "fusion" here...) and will ultimately grant victory. (For instance, the key terrain in the fight with Sadr is... Sadr. He needs killin'). But then the poor bloody infantry need to winkle the surviving dead-enders out... same as it was in the caves of Iwo Jima or the rubble of Berlin or Stalingrad (and in all those battles, the combatants learnt to employ combined-arms teams effectively).
It also sounds like there's a serious need for close-in, anti-personnel armament ("we were fighting out of the hatches with rifles and pistols...") They also need some heavier stuff (probably not an M-1 sized gun, though) -- I don't know what sorts of "short, big guns" might work.
The M-4 is really a pretty damned good gun for this kind of thing but it is still too bulky for a tanker. My CO is a reformed tanker who was at the point of the "battle after the nominal ceasefire" in the last Gulf War. He absolutely swears by the M3A1 greasegun. (Of course, he also took out an RPG team with sabot at 50 meters when his coax was down, so he's flexible).
Unlike the boss, I have only fired .45 calibre subguns at non-human targets, mostly paper and some weird plastic stuff. I prefer the M1A1 Thompson -- but it is still probably too awkward for use from a tank hatch. The M4 is handier and more versatile that the TSMG, but if you're in rock-throwing range it's hard to argue with the terminal ballistics of the .45; and you can train anyone to shoot the Tnompson instinctively in a couple of range days, because the ergonomics (apart from the fiddly safety/selector setup, especially on the early models) are great.
d.o.l.
Criminal Number 18F