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 [citation needed] 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.
So, should we rename halo-5 to halo-5-guardians, and make the first a synonym of the second, or keep it the way it is?