I would love to see it. I think it would be much easier now, as you can develop once and deploy to Desktop, Tablets, Phones and Xbox (also HoloLens and many more).
I think that Epic should support it and it will be much easier for developers to release applications / games much faster on all platforms.
I think there’s a pretty good chance of adding Windows 10 universal app support, which would enable development for Windows Phone.
We’ll have to see once Windows 10 is out, since they’ll be adding DX12 support and stuff like that, we’ll find out what their plans are.
If I’m not mistaken UE4 only supports Windows Desktop, which is different from Windows 8/8.1 “Apps”. I have not tried Windows 10 but I can only assume the same will be true there. If I were to guess I’d wager the current Xbox One implementation is closer to a Windows Phone implementation than the Windows one is.
You are right “currently” UE4 only supports Windows Desktop Applications, but it would be nice to know if they are going to support Windows 10 Apps in the future. Supporting universal apps would enable to publish you games on Phone, Xbox, PC, HoloLens and so on with the same code base.
UE4 already let you do that, UE4 APIs are constructed in the way that your code will build on everything that engine supports, it’s just need Windows PHone support. That Universal App thing only means if Epic gonna use it, it means it will also support seamlessly Phones (as long they also add support for Windows ARM build)