Let`s see:
- Cannot be "Xbox 3", it will be less than PS4.
- Cannot be "Xbox Next" or something, it will be confusing when the next machine is released.
- Cannot be "Xbox 36(insert number here)" like "Xbox 362", it will suggest a minimal increment.
- Cannot be "Xbox 720", people will confuse with the quite outdated 720p video standard, and may imply that the console will only output that max resolution. Similar "Xbox 1080" is risky considering the upcoming rise of 4k resolution.
- Cannot be "Xbox Infinity", there are already lots of products with this name, can be confusing, there is even a new game franchise with it ("Disney Infinity). Even "Xbox Infinite" will be too similar, and also will cause problems to name the generation after.
- Cannot be "Xbox 8", it will fit right now with the Windows 8 brand, but what happens when Windows 9 is released in a pair of years?).
Possibilities: Xbox Loop (sounds silly to me) and... what else?
I am sure MS marketing will come with something clever that we will hate at first but come to love later. Like the "Xbox 360" thing that was quite stupid at the time.