A couple of years ago I would have said a headphone port, but now I record all sound externally.
What I would really love is locked-on auto focus. This is especially important when I am doing everything by myself in less than controlled settings.
4K would be a big positive and I definitely think the 1D X Mark II will have it. What bit rate etc, I do not know and I get a bit lost with all the tech details to be honest.
The ability to easily do slow mo, or timelapse would be another plus, and to be able to shoot a still whilst also shooting video would be good too (in fact if this could be programmed to be done automatically, perhaps at set intervals, that would be really great as I often forget and also do not want to touch the camera to avoid any possible shake).
It would be nice when we can zoom in a little closer when focusing, but what we have now is not too bad.