I expect a new 200-400 will also have FL glass, along with upgraded 300/2.8 and 200/2 lenses (normally the 300/2.8 is among the first to be introduced when Nikon upgrades their line of super-telephotos, it is a little strange it among the last).
The trend among super-telephoto is for them to focus closer with each generation, that is certainly true of the 300/2.8 and 70-200/2.8 which are the nearest equivalents.
Telephotos have tended become lighter with new light-weight materials such as Magnesium alloy and Carbon fibre, also Fluorite elements weigh less than glass.
The trend is also for optical quality to improve - the new 70-200FL is definitely a step up from the previous 70-200 VRII, which was not bad in the first place.
As for a built-in TC, it seems like an "over engineered" solution, but I can imagine it would make the lens very versatile, and purpose-built TC is likely to be better optimised than a generic one.
No doubt, any new version will be expensive!