Following the massive blowback from the Halo community about a lack of splitscreen in Halo 5, 343 Industries may attempt to bring it back for Halo 6. Frank O'Connor made the follow remarks following the community's widespread complaints.
"Splitscreen is fun for me too. It's not something that I object to, and we would've loved to put it in. We'll talk about it for the next game, and we'll talk about it for the future."

Splitscreen was reportedly not a part of Halo 5 because it would not have allowed the game to run at a steady 60FPS.

(via Gamesradar)