In order for there to be a noticeable image quality difference, you have to double the diagonal. So you'd be going from 43mm to 85mm, give or take.
I don't think the lenses would be a huge issue. They could rework some existing designs to throw a larger image circle. A "normal" lens would be 85mm. The job would be easier if they went with a mirrorless design. I doubt the 85 f/1.2 would throw an 85mm image circle, as designed, but I'll bet an 85 f/2 wouldn't be a difficult engineering challenge.
They hardest part would be the business case. a 36 x 48mm sensor would be pointless. The image quality difference would be imperceptible. You'd need a 55 x 70mm sensor to make things worth the effort. But then costs would be too high.
It seems unlikely to me.
Not altogether true you have to view it as "a complete system" and you have to decide what is important a. dynamic range or b.resolution. Sony has managed to improve resolution on its CMOS sensors (include any camera with a Sony sensor) by decreasing the pixel pitch but to compensate has better controlled noise particularly dark current noise which improves apparent dynamic range i.e. that range is truly useable. On a 36X24mm sensor the current happy medium is around 5um however the cinematography favorite the Alexa is around 8um because they want better dynamic range & color slightly at the expense of resolution and that image is projected far larger than most photographs.
The relationship between pixel pitch and line pairs per mm is also crutial ideally you want to match the two so you could be doubling this between small & large pixels hence a "complete system" would aim to maintain the relationship and make improvements to other elements like processing, glass types, light wells, micro lenses, fill factors etc.
The other factor to keep in mind is MF will have less depth of field than FF so auto focus will be far more critical and for bigger lenses speed will be the challenge for AF in sports & other fast moving subjects. Bigger glass means more expensive glass and I agree logic would say do away with the mirror to shorten the back focus and reduce the size.
Canon 5DS, Canon 6D, 16-35 f4L IS USM, 17-40 f4L USM, 28 f2.8, 24-70mm f4L IS USM, 24-105 f4L IS USM, 100mm f2.8L IS USM, 70-200 f2.8L IS USM II, 70-300 f4-5.6 IS USM, 50 f1.8 STM, 100-400mm f4.5-5.6L IS USM II, 1.4EX III, EOS 760D, EF-S 10-18mm f4.5-5.6 IS STM, EF-S 18-55mm f3.5-5.6 STM