Windows 10 is not required to run X-Plane in 64 bit. Do need a 64 bit version of Windows, but X-Plane will run fine in 64 bit in a 64 bit version of Windows 7,8, 10. Think the Windows 10 is a PMDG specific recommendation, not an X-Plane requirement.
I'm suspecting it to be the 767. Ramzzess posted in the shout box after someone mentioned the 767 giving a big hint that would be their next plane. Nothing confirmed of course, but that is the favorite for the next plane Flight Factor will be doing. Whether that is the advanced model they are talking about or pro( or both) who knows.
Also I for one would welcome the DC-6 if that is indeed the plane PMDG is developing for X-Plane.