Well I have an RB and so I have thought lots about alternatives for those rare times when I can't bring 25lbs of backpack along. A camera which is easy to handhold sometimes would also be a boon. In any case if you want to stay at 6x7 or larger then some solutions (none of which I can afford) are:

Mamiya 7 (same or better quality images, interchangeable lenses, lighter, much better for handholding)
Plaubel Makina 67 (super compact fixed lens folder)
Fuji GW670 series or (690 if you want to shoot 6x9) - also fixed lens.