Anything is possible, true. The legacy of Pentax and Ricoh includes:
Exchangable viewfinders, in 35mm Pentax LX and in 6x7
Exchangable back, in Pentax 645
Exchangable lensor, in Ricoh GXR
Exchangable mount, M-mount in Ricoh GXR
Pellicle mirror, Pentax El2000.
There are a lot of possibilities, but I am afraid that they all involve a lot of optomechanical development, which seems to be something that the market trendsetters seem to cut down, as probably the development and production/testing costs might be higher than with electronic advancements and computational "enhancements and corrections".
I see some chance for something that on one hand can help reducing mechanical complexity and offer new possibilities. But as Kunzite pointed out, there are some things to solve, like the avoidance of light leaks from the viewfinder, if, like he pointed out, a construction with the main sensor as the only measuring device would be a future path. If the Reflex is kept it could be kind of hybrid (but not for the recording medium of course), offering another shooting mode without compromising classic DSLR qualities. I can't say if this could support "medium hybridity" by exchangeable backs, viewfinder hybridity by exchangeable tops, mount variability by exchangeable front
If this would be more within the possibilities of a possibly workshop like producer than for mass production manufacturers, is nothing I can judge. I don't expect too much extraordinarity though, to be honest.
Edit: added El2000 with pellicle mirror