What we might see: First footage of new games from The Initiative and/or recently acquired Microsoft developers
This one is a little harder to call because we just don’t know where these companies were when Microsoft made these deals. The ones Microsoft bought could have been winding down from a recent project and didn’t have anything on their plate right away, or right in the middle of developing something and simply switched it to be an exclusive product for Microsoft’s services and consoles.
Maybe Microsoft had specific projects they thought these companies would be good for and put them on those upon acquisition. For example, Obsidian Entertainment still has a third-party multi-platform game coming out later this year, The Outer Worlds. While these companies are assuredly now all working on exclusive content for Microsoft, it’s hard to say if any of it is in any kind of showable state for this year’s E3 2019 presentation.
The Initiative, being a studio that wasn’t acquired but created in-house, seems to be the best bet for showing something of whatever they are working on; but, much like the studios that have been acquired — given that every game being made by these studios is almost certainly set for next-generation hardware (maybe a cross-gen game here and there, but I’d doubt it) — these would all likely be in the very earliest stages of production and therefore might not have anything worth showing in a press conference that seems jam-packed with much bigger things that are further along.
It’s totally possible we will see something from one or more of the acquired studios if for no other reason than Microsoft doesn’t want this to be seen as another Rare situation where the developer just seems to sit around and do rather minimal work for years — like making avatar clothing no one is even looking at before actually making a game.