And I still think the design space of Ranger is big enough that it shouldn't be relegated to sub-class, because its certainly a workable one that RPGs have gotten right previously
It just, 5E cannot seem to get it right.
I think it potentially is...I just think it's a struggle to do in 5E - especially as there's very little room for things that aren't magic to be clearly distinguished.
In 4E they made Two-Weapon Fighting and Archery specific rangery things. But 5E gave these back to the Fighter who is just as good as the Ranger. So the
way they fight can't be the basis for the class.
So exploration? What even is Exploration - I don't think exploration in the sense of the pillar lines up with wilderness adventure - exploration is more commonly meant in the sense of exploring dungeons. (Wilderness adventure is a dubious thing to build a class around as many games won't feature them).
Magic? We already have enough casters and Rangers were never really much in the way of casters before. Gishes? - same problem really. However, Rangers have almost always had some magic so many people expect them to have some.
Skills? We already have a class that's supposed to be all about skills, but they're not just about skills' although traditional ranger skills are overlapping but different so there is some promise there. (Though if we want Rangers to shine at Ranger skills they need either Wis primary, Expertise or both). However Rogues also have a specfiic way in which they fight - they're not just delimited by skills.
The Pet: You could definitely build a class around a character having a pet, but
many people will complain about forcing all Rangers to have a pet. It still seems the most promising thing out of the list so far. And this is probably the only Ranger concept that can't be done with another class.