Officially announced games:

This isn't a game, but the Xbox Scorpio will be formally unveiled at E3.

State of Decay 2

Presumed games:

Sea of Thieves
Crackdown 3

Maybe, maybe not games:

Below: Long in development, still not out, but in theory out soon-ish.
Cuphead: See Below.
Pit People: Is in early access.

Teased/rumored games:

Forza 7 has been all but announced

Ori and the Will of the Wisps was trademarked

The Ashen developers mentioned that their game would be shown again soon.

There's something "small" related to Halo that is not Halo 6 and not Halo 3 Anniversary

Additional notes:

Microsoft has a marketing partnership with Middle-Earth: Shadow of War. It is always plausible that marketing partnership games show up on stage.

Studio Gobo signed their partnership with Microsoft a short while ago. It's probably too early for their game unless they had been working on it for a long time, and only just recently announced the partnership.

It seems too early for Gears of War 5, and Halo 6 is already confirmed as not showing.

I could really use your help filling this one out. I admit my domain knowledge here is not what it used to be.