As seen on Justia Trademarks, Microsoft has filed a new trademark called, "Halo: The Endless". While there isn't much information to go off, the title alone suggests that this could be a story expansion.
SMALL STORY SPOILERS AHEAD
As you progress through the story, a new race of beings called "The Endless" are brought up. The species are tucked away by Zeta Halo, but the Harbinger is trying its best to unleash the deadly beings. The group is continuously teased while remaining minimal in their presence throughout the campaign.
Given the title of the trademark refers to the species, it's highly likely that this could be campaign DLC that involves The Endless. Seen as that Halo Infinite is a sort-of rejuvenation for the franchise, with multiplayer and story extensions planned for the future, The Endless could be the first step in this direction.
Of course, this is all speculation. 343 Industries is hard at work developing content for the sci-fi shooter, so we shall wait and see what lies ahead for Master Chief and co. We'll keep you informed of any new updates and news.