I can't really think of anything Nikon could bring out that would make me switch.
But then I didn't think I would be sinking a ton of money into a Sony system until they brought out the A7Rii, either.
I'm with Cooter on the need for stills/video convergence. Any modern system with the readout speed for LiveView has the potential to record video. If you're going to do it, do the best job of it you can. Even though stills camera ergonomics aren't ideal for shooting video, a bit of thought and firmware can go a long long way to helping (peaking, audio levels, RGB histogram with clip warnings, log gammas, log gamma with R, G, B channels encoded separately so you can white balance in post, suitable video modes for AF and IS, etc.) I'll be interested to see how good a job Hasselblad have done with video on the H6D.
If I'm going on a stills shoot I take the Hasselblad.
If I'm going on a video shoot I take the RED.
But I take the Sony A7RII to every shoot, every time. I need to travel light, I'll take just the Sony. (OK so I also throw in a 7D or GH4 and a boggo standard zoom since I have them lying around, so I have a backup. When Sony comes out with an A7Riii I'll buy that and use A7Rii as backup.)
Two cases versus five makes a hell of a difference flying out somewhere to shoot, and even in the UK can make the difference between taking my car and needing to hire a van.
Now if only I could get continuous lighting kit that would genuinely do for both highest-quality stills and video, I'd be super happy.
Maybe Nikon should start making really, really good lights :-)
Cheers, Hywel