Halo 5: Guardians was launched 3 days ago, on 27th October, 2015.
We already have a tag for it: halo-5, which (at the time of writing) has 12 questions.
According to the official site and the Microsoft XBOX site, its official full name is Halo 5: Guardians. It is written with its full name in both of the website addresses, and all the websites on the first page of a Google search for Halo 5 also refer to it as Halo 5: Guardians.
AFAIK, our current policy  for game tags is to use the full official game name where possible, and propose common names as synonyms. According to this policy, the tag should be named halo-5-guardians.