Yep, it will have a number and a subtitle. How innovative! 
Here's the official announcement from Bonnie Ross, General manager at 343:
Xbox Wire
Teaser poster:

Here's the official announcement from Bonnie Ross, General manager at 343:
I’m happy to reveal that “Halo 5: Guardians,” the next installment in the legendary saga of the Master Chief, will launch on Xbox One in fall of 2015. In the tradition of every “Halo” game since its debut in 2001, it is a massive and exciting project.
“Halo 5: Guardians” is a bigger effort than “Halo 4.” That applies to the content and scope of the game, as well as the technology in what’s now a brand new and more powerful engine. Certainly there are some core elements carried over from prior games, but we’ve invested a huge effort in retooling our tech to take full advantage of the Xbox One’s hardware and ecosystem to create worlds and experiences worthy of next-gen.
It’s a game that will hopefully demonstrate the talent, learnings and abilities of the 343 Industries team. A game that will incorporate the things we learned from “Halo 4” about technology, aesthetics, performance and scale – and perhaps more importantly, understanding and embracing a community of gamers who love what lies at the heart of this game, and the limitless potential of the “Halo” universe.
Xbox Wire
Teaser poster:
