I think this is a somewhat confused question: as xiota has answered the important thing is whichever of the flange-to-focal-plane distance (FFD) & the rear-element-to-focal-plane distance is smallest. This is the reason why, for instance, adapted SLR lenses stick out a long way on mirrorless cameras (the rear elements of SLR lenses must clear the mirror, which is not the case for dedicated mirrorless lens designs).
It's possible to see this by simply comparing FFDs for different mounts. There is a convenient table here.
- Fuji X mount is 17.7mm
- Sony E mount is 18mm
- M4/3 is 19.25mm
- M mount is 27.8mm
- Pentax K is 45.46mm
- Nikon F is 46.5mm
- Canon EF is 44mm
It's immediately apparent from this that, whatever you do, you can't mount both a lens from the short end of this list and a lens from the long end of this list without one of them sticking out quite a long way from the body, unless you do something absurd like recess the mount flange into the body by many mm.
Even if this were not a problem, simply moving the sensor backwards in the body of the camera is also extremely nontrivial. Sensors are thicker than film was, and have support electronics which has to go somewhere. Almost all cameras also need to fit, behind the sensor, the screen which goes on the back of the camera, which also needs support and electronics. You can see the differences by comparing camera designs which have the same mount between film & digital bodies. A good example is M-mount cameras:
- the M3 is 33.5mm thick;
- the M6 is 33.5mm thick;
- the Leica CL is 32mm thick, as is the Minolta CLE;
- the Zeiss ZM is 32mm thick.
It looks like the thinnest a film M-mount camera can be is about 32mm: I suspect the Leicas are slightly thicker because they're more robustly made. Comparing this with the FFD for M mount above there is not much leeway here.
Now look at the digital Ms:
- the M8 is 39mm thick (not full-frame)
- the M9 is 37mm thick
- the M10 is 39mm thick
The mount flange on a film M-mount camera is essentially flush with the front of the camera, so the digital Ms are not thicker because the mount is being moved back further into the body: they're thicker because the sensor, the support arrangements behind it and the support arrangements for the screen take up more space than film did. Looking at the above dimensions we can guess that that 'more space' is somewhere around 5-6mm (and we can also deduce from the M mount FFD that the film, pressure plate & back of the camera on, say, an M6, is a little under 6mm all in: it's fairly easy to confirm this is about right for most film cameras by opening the back of course, & measuring how thick film is).
Now of course, we don't know that it's not possible to make things thinner than that. But I think it's pretty clear that if Leica could, they would – there have been many negative comments about the size of digital Ms. An we know that they are getting sensors which are customised to their requirements (since they make cameras with monochrome sensors), so they are certainly talking to sensor manufacturers. So making the thing thinner is at least hard.
Putting this together we can see a couple of things.
- It's not possible to mount lenses with widely-varying FFD distances on the same body without either one of them having a mount(-adaptor) which sticks rather far forward from the body (or recessing one of the mounts far into the body, which would I think be absurd). In the case of, say, E-mount, there is a difference of 26mm – more than an inch - between its FFD and the Nikon FFD.
- Sensors & their support arrangements (including the back of the camera, screen &c in this) seem to be about 10mm or so at least.
- A camera which can mount a Nikon SLR lens with the mount flange flush to the body would therefore need to be about 54mm thick.
It's then slightly hard to find dimensions for cameras since they often include various grips &c. However the Fuji X-Pro 2 is claimed to be about 35mm thick at its thinnest, which looks like slightly thicker than it needs to be (compare the FFD for its lenses, 17.7mm with the FFD for M mount lenses of 27.8mm and remember the M9 is 37mm thick and you can see how much work Leica must have done to make things thin). But to mount a Nikon SLR lens on an X Pro 2 with the mount flush to the front of the camera the camera would need to be nearly 20mm thicker. That's probably not a tradeoff anyone wants to make.