Halo was first teased at E3 2013 this year, the title was simply, "Halo", instead of "Halo 5". A new, updated version of that teaser, produced by Axis Animation, now features the title Halo 5.
It's not as obvious as one may think - next-generation games have had the habit of dropping their numbers in a sort of quasi-reboot. Such as Thief 4 becoming Thief, Dragon Age III: Inquisition becoming Dragon Age: Inquisition, and Killzone 4 becoming Killzone: Shadowfall, and so on.
This indicates that the storyline introduced in Halo 4 will continue to Halo 5 unabated. No release date has been announced for it.
Axis Animation had posted the revamped video on, but the trailer has been pulled.