Xbox
Xbox 360
Xbox One
Xbox One X
Xbox 5
I believe that Microsoft has a good case to call their next system, Xbox 5. An original idea was to call Xbox 360, Xbox 3, and have it stand for three pillars to make sense of the name, because they didn't want the platform to sound inferior as Xbox 2, to Sony's PlayStation 3.
The did something similar to the three pillar concept, but of course went with Xbox 360, as in "all-in-one" piece of hardware.
Xbox One is once again, kind of an "all-in-one" sounding name. The Xbox One X is four years removed from the original Xbox One, which is the same amount of time between their first system and its successor, so both timewise and hardware improvement-wise, I think they have a case going for them, to somewhat retroactively consider the Xbox One X as their fourth piece of hardware.
Unless they come up with some different theme, I think they were just called Xbox 5.