Mercurius
Legend
The catch is, this is already happening with Necromancer Games, ENworld, and others. But 3rd Party Products - even good ones - are not official enough.
After all, the 3PP Fifth Edition Foes was developed and converted by Steve Winter, an ex-WotC employee who was with D&D since 1st Edition and was working for the brand as late as 2013 and co-wrote the two Tyranny of Dragons adventures. It's pretty darn close to "official" mechanics-wise.
The problem with 3PP stuff is, as I see it, two-fold:
One, much of it is now PDF only, and some of us old fogies (grogies?) don't like PDFs (although I don't mind it for short black-and-white adventures that I can print out).
Two, official material implies a certain level of quality, both in terms of content but also the physical product. I'm thinking also for those who like splats, "officiality" has its own perks.