Officially announced games:

Assassin's Creed: Origins
Far Cry 5
The Crew 2
South Park: The Fractured But Whole
A New IP (see discussion below)

Teased/rumored games:

Mario x Rabbids: Kingdom Battle: It's not entirely clear if this will be at Ubisoft's show, Nintendo's show, or both
The Division: Ubisoft has been saying they plan to unveil The Division's Year 2 content soon

Ubisoft's New IP discussion:

It's not impossible this is referring to Mario x Rabbids, however, Ubisoft has noted that they have an online centric new IP coming next fiscal year that was originally coming this fiscal year, and Mario x Rabbids is not an especially online centric game.

Ubisoft has a new space exploration sci-fi game in development code-named Pioneer that was rebooted last year, but was teased in Watch Dogs 2. It wasn't explicitly stated this was online centric, but it would be kind of odd if it wasn't. It could be ready to show by now.

Ubisoft has surveyed about a new online centric pirate game that would follow up on the core concepts of Assassin's Creed 4.

Ubisoft has some kind of game being built with Studio GOBO, and I believe both of their new games are online centric based on job postings. This was a third person action adventure title focused on multiplayer combat.

It's always possible there's something else I missed here. Ubisoft makes a lot of games.

Additional notes:

Ubisoft's online centric Avatar game (by The Division people) will not be at E3 as it is not releasing until at least 2020.

It's probably too early for The Division 2 or Watch Dogs 3.

Rainbow Six Siege is not planned to get a sequel until well after Ghost Recon Wildlands gets a sequel, according to their last fiscal call.

Ubisoft has been rumored to be working on a new Splinter Cell, though it's not entirely clear how development on that is going, as we haven't heard anything in a year or so.

Ubisoft has said Beyond Good & Evil 2 will not be at E3.

Aisha Tyler will not be hosting this year, in favor of having developers host.