Halo 2 came out in November 2004, Xbox 360 came out November 2005...
They could have a Halo game ready for launch even if they release Halo 4 for te 360... Halo Wars 2 or something similar to ODST perhaps, or better yet get some new IPs out at the launch of the console to help establish them and leave Halo X to release a year or two after the Xbox 3 because it is a safe bet that it will be a big seller.
Personally, as far as naming goes, I would like to see them go with the 'Fusion' suffix and could push interconnectivity between Windows, Xbox and Windows Phone as the big selling point like they were meant to with the 360.







