TL;DR
Universal Apps across Xbox and Windows and Windows Phone for gaming and apps will be announced.
What I believe will happen:
Microsoft will talk about Universal Apps on Xbox One. These Universal Apps are apps that can run from a Windows Phone, to Windows Tablet, to Windows PC, to Xbox, to Microsoft PixelSense (look it up). Microsoft already has universal apps that run on pc and phone (Halo: Spartan Assault is one of them). It lets the developer create one big code base on one platform and with minimal code changes for other platforms. Khan Academy is the first universal app that is on the Xbox One (if I remember correctly). These universal apps are bought once on any of those platforms and a "free" version on the others.
Microsoft will open up development (more on that at //build/ 2015 conference later in the year) to the store (the Microsoft One Store) that will be available on the Xbox. That means that those apps (or games...granted not AAA games...yet...that may be announced at //build/ along with each Xbox being a devkit) will have 3rd part devs on board. Basically creating the appification of TV, the thing that every company has been trying to do. What Microsoft has been trying to do with Xbox from the beginning, which is put a PC in the living room. This can possibly revolutionize the way we interact with TV. We'll have TV apps that we can navigate and use with our voice or controller or gestures. Think about companies like Nest with home automation, Dominos that's already making an app for the UK and I'm sure soon the US (Xbox "Feed Me"). It has that type of potential.
Windows gaming will be through the Windows Store. Phil Spencer may very well say that developers will be given the opportunity to make their Universal Game Apps play across Xbox Live to Xbox One players but that is up to the developer. The game section of the Windows Store won't be called games but Xbox as Xbox is a gaming brand. It will start with the release of Windows 10 but it will take time before those games that are out (or coming soon) to be universal apps.
This presents a unique opportunity for Microsoft that no one else has. They can leverage the ability to buy PC games through their store and whenever a person decides to get an Xbox, "all" of their PC games come with them, or vice versa. Let's say Killer Instinct does come out on PC, I already bought the game and all the DLC, I can play it on my PC as well because it was already bought on my Xbox. This also helps with the ever growing market of digital sales. Buying a digital game, you at least know that you have two copies of a game instead of one. There's more incentive. Also and this is the biggest thing in my opinion, if the next generation Xbox runs on Windows 10 code base (which I believe it will), then backwards compatibility will forever be there. The biggest problem when changing generations has been solved and makes it easier to buy into the next generation of gaming than it has in the past and a huge leg up on Nintendo and Sony.
What I believe what won't happen:
Partnership with Steam. This won't happen because Microsoft is changing the way they are going to monetize on Windows. They want to make money through apps (as one way). Gaming gives the highest payout for apps. Partnering with Steam would diminish the return. They have to come up with ways that they can compete with Steam, it won't be great like Steam out of the gate (just as Steam wasn't great out the gate) but the crossplay with Xbox will make it enticing.
DASSIT