I am currently on my way to Seattle for the #MSBuild conference and figured as a pre-build step (lol, right?) I could take a look into the crystal ball and make three predictions of what will happen at build and what announcements we will see.

1. WebAssembly / Blazor

D'uh ... that's an obvious one, but nonetheless I'll call it here. Since there is a great hype around Blazor currently and the ASP.NET Team seems very eager to jump onto that, we will definitely see a strong focus on that topic.
I predict that Microsoft will show serious commitment towards WebAssembly and will officially announce future support for the Rosyln toolchain (vs. Mono currently) with WebAssembly.

2. HoloLens 2

A lot of time has passed since the first HoloLens prototypes and devices have been shipped and technology has moved forward since then. Also there hasn't been much noise from Alex Kipman and his team recently. So I am pretty confident that HoloLens has gotten a significant upgrade recently which will be demoed and announced at the conference.

3. Electron and .NET

I am going out on a limb here and predict that Microsoft will embrace Electron as their future cross platform UI technology, since that is where everyone's moving currently and UWP is basically dead in the water.
At least we should see Electron being integrated into the Xamarin stack somehow.