Most major games get announced long in advance of their actual release so the hype builds early. However, Xbox head honcho Phil Spencer just teased an announcement of a game we don't know about, that could be here very soon. The tease comes in the form of a tweet, with a link to game list that has very specific parameters.
The list that Phil Spencer links to in the tweet is an IGN piece about multiplayer games that are highly anticipated in 2017. If Spencer expects to add to the list then we're talking about a game that he thinks would be worthy of being one of the "most anticipated" and we're also talking about something that would be expected to be released this year. A game that fits those two parameters we would have expected to hear about before we now, but apparently that's not the case.
The other item that we can almost certainly add to the list of specifications for this title is that it's an Xbox One exclusive. Otherwise, it would be unlikely that Phil Spencer would be touting it so much. He says "we," clearly referring to Xbox. He could also be referring to Microsoft Studios specifically. Odds are this is a first party title, but that's not entirely clear. It could be that this is a multi-platform release that Xbox just has a deal with that will allow them to announce it.
So what game could this possibly be? Xbox certainly has a franchise that meets all of these specifications, but it's highly unlikely that a new Halo game is within a year of release and nobody knows it. A sixth game in the franchise is as confirmed as it can be at this stage, though it's never been officially announced. It's technically possible that could be the answer, but only technically.
Xbox's pirate-themed multiplayer game Sea of Thieves is expected to be released this year. It doesn't have an official release date, but Phil Spencer can't be referring to it because that game is already on the list in his link.
Is there a new multiplayer IP on the horizon at Xbox? One has to assume that the studio is working on several different projects that we don't know about. A new IP is also the most likely thing to fly under the radar since nobody would know to look for it.
At this point, we have no idea when an actual announcement will come. It could be any minute, it could be at E3. We'll be keeping our eyes open and let you know when we know something.