Well, I don't think it should be a new game. If you make it a new game, you might as well throw it on the small pile of games that have gotten a curiosity glance and a few purchases, but no long-standing growth. If you make 5E's version of Monte Cook's Arcana Unearthed... it will have the staying power of that book. Which is... almost nothing.
Whereas if the book actually does what it sounds like it's supposed to-- take everything in the Player's Handbook as fact and then build and expand upon everything... make each part more comprehensive, potentially more elaborate, and most importantly able to be added piecemeal... then maybe you might have something. The DMG has all manner of Variant Rules in their books... but little to no explanation on how these building blocks can be used and incorporated to create specific types of gamestyles or gameplay. You would hope that A5E would do that. Make these Variant Rules packages bigger. More expressive. And then as part of a section in the book, use all those rules you made to build combinations to be able to actually recreate truer versions of "3E" or "4E' or "Basic" or all the versions of D&D that WotC originally said their intentions of 5E were to do. If you can accomplish what WotC originally suggested they wanted... then you might have something long-standing because you'll have a whole huge swathe of players who will thank you for it.
But if it all just ends up being nothing more than a crapton more subclasses and class "fixes" but written up as part of an entirely new "Player's Handbook"... that stuff would be no different than any corrections, additions, and fixes we can already buy individually on DMsGuild right now. And we wouldn't have to spent the extra $50 for the re-writing of huge swathes of the current PHB so it can be "it's own game" to get it.
