New 20MP FX sensor (not sure)
New focusing module with larger coverage and 153 autofocus points (confident)
Native high ISO of 102,400 (confident)
15 fps (not sure)
Body design similar to the D4s (confident)
4K video 60/30fps (not sure)
Full HD slow motion (not sure)
Announcement expected in early 2016 (confident)
Read more on NikonRumors.com: https://nikonrumors.com/2015/12/11/the-new-nikon-d5-will-have-153-autofocus-points-and-native-high-iso-of-102400.aspx/#ixzz3u8wFBPi1
The Nikon D5 has a 20.7 megapixel FX sensor in it. This is fact (CR3).