Well, this is pretty sad: In an interview over at Eurogamer, Toshihiro Nagoshi said that the series probably will never head over to the Xbox. It seems this is partially motivated by the fact that Microsoft gave them the cold shoulder before:
At the beginning of the project we spoke to all the platform holders including Sony and Microsoft, every platform holder was negative about this prospect, but we kept on pushing because we believed in the potential, and as we kept going one platform holder that showed interest and saw the prospects of this title was Sony.
He also adds that Yakuza of the End will be the last game in the franchise to have that kind of gameplay-style.
The other motivating factor is probably that the Xbox has zero life in Japan, and the game is hardly a huge seller outside of Japan, so it makes little sense to invest the time into porting it if the returns will be so little. Who in this era doesn’t own both the 360 and the PS3 by now anyway? When the console generation is as long as this one has proven to be, I can’t really see that being a big factor.