I think it'll either start at a slightly lower spec or a slightly larger price so not to cannibalize too much of the iMac Pro. I suspect it'd be a dual socket machine, so Xeon W is not an option, leaving E5 or E7 as possibilities.
It'd be awesome if Apple pulled off the technically amazing feat of having a Xeon Phi as the only CPU, but I am not sure it could even be done (and with no virtualization support for it, if it's even desirable). Still would be cool to a level only Apple uses to do.
$5000 is about the right ballpark.