OK, it looks like the rush-job that was Windows 8 has finally been beaten into something that is usable. My first suggestion for W10 is to allow live tiles on the desktop.
The Universal Application API also sounds good. I assume this is only for W10, but if the hints of free upgrades from 8.x to 10 are true then that's good news.
As a developer the big, huge bonus would be for MS to back-port that universal API to Windows 7 and make that part of Windows 7 SP2. Some of my clients are only going to be completely rid of XP in the next few months (and installing Windows 7).
Our product(s) are targeting Windows 7 (and Server 2K8/R2) as a minimum requirement. That means Win32/.Net which is supported on 7/8.x/10.
Unfortunately we can't migrate to U-API since we must support Windows 7.
What are the chances of U-API being back-ported? Probably nil. This will be MS's final hurdle to clear - the humongous installed base of W7. I suggest that until they "get it" and backport U-API to W7 their app store will continue to flounder.