This is for a backwards-compatable-but-with-substantial-changes 5.5E or 6E book, in my opinion. Has Level Up 5E and other books, along with potentially Avatar's Kickstarter success, lit a fire under WoTC's ass?
I don't have much D&D experience but from reading quite a bit across various forms, reading features, and the 30 or so session experience I've had, I did make a lot of points on the Martial versus Spellcaster divide, what could make Spellcasters much better, and the issues with several PHB subclasses (in particular that Fighter needs Battlemaster in their base class and Champion needs to be dumped, Beserker is poor, etc.)
Exciting times ahead!