With the introduction of the Leica M sporting both an optical (actually coupled rangefinder) and the option to fit a EVF that works very well together, and with rival Sony offering electronic finders in their SLT cameras, what can Nikon and Canon do with future pro oriented models? Easy, should on-sensor phase detection AF (such as that found on the new Canon EOS 70D) become fast and reliable enough I can think of several solutions. But the one that appeals to me most is a return to the interchangeable finders of their film-era cameras. While the maker's primary concern will be the ingress of dust, how difficult could it be to offer a single body with the option of optical and electronic finders?
The reflex mirror and focusing screen could be retained for the optical (pentaprism) finder and finder based PDAF systems*, and be simply locked up when the electronic finder is attached. You wouldn't be able to switch seamlessly as you can with the Leica M, but that's inherent with the CRF design (and ocular occupying one corner), however it would instantly quash detractors of either system and allow the user to choose the appropriate finder for the job in hand. It's not a new idea, I've been thinking about this for a while but was reminded recently when using my Nikon F5 and again when reviewing the new Hasselblad H5D (pictured above) for a magazine. I've not seen this mentioned anywhere else in print or online, and don't like to make such thoughts generally known on the net, so remember where you read it first.
* The pedants among you will know that the finder-based PDAF systems are for the most part based in the mirror-box (and use a secondary silvered mirror slung below the main reflex mirror to direct the light to the AF sensor). But it's common to call it a viewfinder based system, as that's how the user perceives it.