My idea is to alter today's main battle tank design (the standard 4 crew designs) in the following ways:
-General updates/improvements that would be applied to all new vehicles (better engines, active defense systems, better computing and communications, stealthier design, etc.)
-Make all armor beyond that capable of withstanding small arms fire removable/modular
-Give the tank a diesel-electric drive with the electric motors in the wheels such that the tank can operate without tracks.
-Give the tank water impellers for amphibious operations.
-Move the turret to the rear of the tank.
-Remove all crew compartments (and add autonomous capability, although fire commands would still be from an outside authorized source, and there would still be manual override controls mounted on the gun)
-Lower the Roof of the turret below the gun, such that the entire tank, other than the gun, is no taller than the tracks
-Allow the gun's vertical pivot to itself be raised or lowered. It would be lowered so that the entire gun could drop down into the hull, somewhat similar to the Zumwalt class destroyers. It would be raised to increase gun elevation (how far up the gun can be pointed without the back of the gun hitting something below it) for indirect fire or antiaircraft fire missions using guided munitions.
Keep room for 1 crew member in the hull for consistent manual override capability.
Dual Weight Class
-Putting MBT tracks on a much lighter system might seem like a total waste of weight and cost, but the size and number of the road wheels allows it to operate with good mobility without the tracks, which increases speed and fuel efficiency. The relatively very high strength suspension would allow the vehicle (without humans on board) to be airdropped from low altitude without a parachute. The exact maximum altitude for this has yet to be determined.
-Quick field maintenance allows you to turn what you just airdropped or drove onto the beach into a MBT.
-Instead of having to follow up light vehicle deliveries by delivering entire heavier vehicles, you only have to follow it up by delivering the armor, tracks, and possibly a couple other components, which is much easier than delivering the entire vehicle, since it's lighter.
-In an emergency situation, this MBT could detach its armor to prevent having the tank sink (This happens, a M1 Abrams accidently drove off a bride during operation Iraqi Freedom, the tank was effectively lost [although I'm sure they salvaged it later]) or be stuck at the bottom of a river after a failed attempt at fording. If within reach, winching the armor out of the water sometime after would be relatively simple, since you could do it one piece at a time.
-One small benefit is the ability to equip the tracks without equipping most of the armor, thus making a vehicle which is able to drive at nearly full speed over snow and mud.
-It's hard to predict how much heavy armor the military will need decades from now, so this vehicle allows you to adjust your ratio of heavy armor to lighter/more mobile vehicles from conflict to conflict, or even from battle to battle, as enemy weapons, active defense technologies, and budgets change.
Limited Crew Protection
The benefits are pretty evident: A lower profile and stealthier tank and ample room for moving while reloading.
The immense amount of room to load the gun allows the number of loaders to be adjusted anywhere from one to however many are required to fire at the gun's max burst RoF.
In very safe environments, which might be encountered when the vehicle is acting as a self propelled howitzer, the vehicle could be crewed by humans, however,
In most dangerous environments, the gun would be loaded by robotic soldiers, who would be relatively well protected against small arms fire and nearly impervious to NBC dangers.
A gun shield would protect the loader(s) from direct fire from the target.
When an incoming large artillery shell or top attack missile is detected, it's possible that one or two of the robotic loaders would hop into the gun stowage compartment, and thus be protected from the attack.
In general the dangers of large explosive warheads making their way over top of or beside the tank would be greatly reduced with an effective active defense system installed.
In the event that the crew is wiped out, and replacements are not available, the tank is able to load the gun itself, but the loading mechanism is relatively simple and compact, so the gun must be nearly completely lowered, such that the rear of the gun is in the turret, and the rest of the gun just barely clears the hull. After being loaded the gun must be raised such that the recoil won't cause the gun to collide with anything.
Note: The tank would be able to serve as a charging station for robotic soldiers, so there would be no real need for robotic loaders to have any large batteries on them. Also, if recharging takes time, then this vehicle wouldn't need a designated crew, but instead would be crewed by whichever robots are being recharged.
^This is assuming that the robots aren't powered by internal combustion engines, which would probably be unwanted in a robot designed to operate in buildings and vehicles (just like people) due to noise and fumes.