I'm not sure if this is legal or not, but if they made an iOS and Android emulator so you could run both iOS and Android apps on the Windows phones, some people might get a Windows Phone then who'd otherwise be getting one or the other because they figure they get all types of compatibility.
Then I'd make backwards compatibility to Windows Vista where all windows versions could run: Windows Phone apps, iOS apps, and Android apps. It is very very important to do this similar to a sandbox where the apps can't escalate privileges and whammy system files like the old Windows .exes do.
Then suddenly you not only fixed the windows phone, but the #1 problem with the Windows operating system in the Internet age. The windows operating system is just not designed to download random .exes with wild abandon like it should. Part of the reason Windows lost share to Apple in the late 90s is because Windows was targetted by viruses more. If you're doing sandboxed apps as the future of Windows, suddenly it is very very hard to get a virus all of a sudden. Windows users would rejoice with having Windows Apps they could run on their PC without worrying about being hosed. Windows could even allow an app data backup cloud, so in the case you do get a virus in the old ways, when you reinstall your PC, all the apps can reinstall with your saved data. Just being able to explore downloading lots of free app software without worrying it making your computer die would be awesome. It is what we should have had with Windows 98 if Microsoft took the hard road and completely redesigned their operating system that an .exe couldn't make changes outside its install directory and other virus resistant moves. Instead of downloading every piece of cool looking software on the Internet, the slight chance it could be a virus means we just roll with the software we need to use.
I think a lot of Windows users would love to be able to use Android and iOS apps also. Throw in the weird functionality of a Surface pro they're trying to market of being a touch screen PC, and running Windows/iOS/Android apps on them only make sense.