The modern metro UI should stay EXACTLY the way is it, as it has proven to be a winning combination! Three percent market share! Keep the current design language and scare off dev's even further from a distance third place mobile OS. The thing that's so great about the current (dated) app design language...it stands a great chance of persuading even more large companies to pull their mobile apps (see Chase Bank).
I don't know what it is with you guys. The Windows Phone 7/8 "metro/modern" app design is really dated in 2015, it looked great when it debuted, but it's time has past. And with the position Microsoft is in, they are the last one's that should be going against the grain when it comes to UI design ---- 1) They are 3rd place in the mobile space 2) They tried in in 2012 and they delivered the failure known as Windows 8. If you want Windows Phone to succeed, welcome & embrace the change. We're not getting the old design back, so instead of complaining, suggest ways to make sure the updated design language provides a great experience on mobile devices. Microsoft can't continue down the road you guys want them to, or you'll be forced to use iOS or Android in a few years because there won't be a Windows Phone OS any more.
Besides...it's not like we're losing live tiles and the actual OS level UI.