A whole sequence of tweets on the subject can be found at:
https://www.sageadvice.eu/2017/10/11...ificer-mystic/.
So it looks like they will focus on the ranger before the mystic and artificer, "when the new options in the UA ranger become official, using them with other official material will be seamless", "In the end, there will be 1 ranger: the PH ranger. It will get alternative features. The official game won't have 2 versions of a class", and "You'll be able to take feature X in place of feature Y. It won't be a whole different class."
Sage is great for ideas and variants, Sage advice was a medium for gamers to ask questions from
TSR and WOTC and getting ideas generated back for players in Dragon magazines for ages.
The Mystic and Artificer where concepts discussed and presented in Dragon Magazines decades ago.
Along with Hedge Wizards, Herbalists sub-class, Librarian and tons more.
Alternative features are the possibility of exchanging Feats, proficiency and skills that might fit a players
choice in developing a Ranger from extreme regions or other-worldly environments, all makes sense if
the concept fits a world the player is planning on playing in and the DM of that world reviews, discusses
and maybe tweeks that character to better fit his campaign and or world.
An old adage was repeated for decades and I think it applies here:
"The rules are guidelines and are not set in stone".
The exception would be for AL and tournament play and submissions from creative authors making new
modules or new game material submissions to WOTC.
Just food for thought....PEACE