Reports are that Mike Colter (Locke)
let it slip that Cortana will appear in Halo 5, but he didn't specifically say she'd be reprising an active character role (so we still don't know if she's gonna stay dead). He also confirmed that Locke is on a "manhunt" for the Chief, which doesn't sound pleasant. This fits with the idea that the Chief has gone AWOL and is in serious trouble with the UNSC, even possibly being viewed as a traitor a la #HunttheTruth. Finally, Colter also confirmed that Locke will be in more than one future Halo release.
Basic setup seems to be that Chief has gone to go do his own thing, Cortana's death is involved, the Forerunner threat is still around, Halsey and the new Covenant are involved, the Arbiter is back, and Agent Locke, now a Spartan, is hunting the Chief.