I'm curious about this requirement? (I'm a daily MSDN-licensed user as well). With VS2013 you can also license by signing in with your MSDN account instead of providing a license key. Are you sure you can't just use this approach to continue on VS2013?
Having said that, we are in the process of certifying for the new tools and platforms. If you require the manifest editor and deployment tool, this won't install into VS2015, but if you don't need this, you _should_ be ok (but again we are still working on certifying so we can say we officially support it). I've been using VS2015 for a while and haven't seen any issues personally.
Wrt to .NET 4.6 that is still pending certification. According to Microsoft it's supposed to work with very few gotcha, so you're probably fine. Again we haven't certified yet but working on it. Same goes for Windows 10 Universal support. We are in the process of certifying this. We know for sure Windows 10 Mobile won't work (not supported to use native 8.1 SDKs in a WinMobile10 app), but so far it's looking good for Windows 10 Universal Desktop apps - in fact the upcoming ArcGIS Collector app for Windows is a Windows 10 Universal app, and the team haven't reported any issues here either. Keep an eye on our blog in the coming months for official word on whether it's fully supported.
The major Quartz release coming next year will fully support all these things.