An optical viewfinder with an optional EVF attachment would be very interesting - especially if paired with the right software. We all know there are certain subjects and conditions where EVF does not work - and the OVF would be fine. But there are also capabilities of an EVF - such as increased accuracy with contrast detect AF, the ability to zoom and focus or check focus, the ability to see other information. Think about products like CamRanger - essentially using a smartphone or tablet as a viewfinder and control. Or the ability to apply creative effects - like picture controls but expanded - in real time to stills and video. And of course, for video an EVF would be a nice benefit. Nikon has already delivered an EVF as an accessory item, so it would not be at all surprising to see it paired with a DSLR.
If you are using an EVF anyway, the mirror is already up and does not need to move. Why not deliver a high frame rate - such as the 30fps or even 60fps bursts of the Nikon 1 system. The D5 will have a large enough buffer to handle 30fps at reduced size - 6mp or so. A 2 second burst at 30fps would be pretty remarkable for sports or wildlife.