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There was a German-American project for a joint main battle tank development in between the Leopard and Leopard 2 development. It was too technologically ambitious, too expensive and the two countries could not agree on a common approach or even type of main gun. The project failed by about 1970 already (the Soviets were much more successful with their technologically daring tank project, which became the T-64).
http://www.panzerbaer.de/types/bw_kpz_70-a.htm
The turret is so huge for a serious reason: The driver was in the turret. This solves some problems (especially how he gets out through the hatch real quick in case of fire and main gun just above the hatch) and added huge problems.
I suppose that modern technology with all-round cameras has largely overcome those added problems, albeit maybe not for all kinds of battlefield vehicles.
A self-propelled gun (SPG) is a category of vehicles that does not require much line of sight combat capability, especially not on the move. So I suppose a tracked (nowadays continuous composite bandtracks) SPG could make use of a driver in the turret. SPGs have the gun in a certain travel position (usually straight forward) during almost all movement anyway.
The further crew could be two men for loading (one for shells, the other for propellant modules). Manual loading with just some power ramming is fairly cheap (unless you insist on active duty personnel all year round), very reliable, low maintenance, very adaptable and there's more personnel for maintaining and securing the vehicle than with a more or less extreme autoloader concept. The driver would be busy driving while on the move, but could be the commander while in firing position. One of the loaders could watch the flatscreens and make decisions for crew on the move. A fourth person would increase volume and weight for allow for a conventional (permanent) commander function and would also add much (but likely not important) combat capability on the move.
Drive-by-wire was extremely ambitious during the 1940's to 1960's, but today it's nothing special any more. A driver in the turret could steer the vehicle easily with drive-by-wire.
What's the benefit of having a driver in the turret? You get rid of some ergonomics issues (see the driver escape hatch problem of Boxer SPG) and the vehicle can be a lot (almost 1.5 m) shorter. The vehicle would be reduced to frame, suspension, wheels, bandtracks, a front engine compartment and a big turret. A shorter vehicle is a lighter vehicle, but it would also be less comfortable on rough ground (more pitching movement).
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