If only someone figured out a way to create a sensor suite that could be fit into custom film doors (for those SLRS/cameras that have them, sorry Leica M) I could imagine a modular system allowing you to buy a sensor that snapped/installed into different back doors for use with a variety of SLRs/RFs.
3-D printing could also solve the problem of creating the variety of backs the world would need. If Nikon managed this trick, it would have far outsold the Df.
Then again, building something like that with a FF sensor and all the necessary electronics is no small task.
But the eternal optimist in me always thought it would be a matter of time till we saw a kickstarter for such a thing.