I applaud your hard work, that shows through. The discussion about class pillars and how that interacts with class features I think is spot on, even getting reinforced with the designer sidebar about "Ribbons" in the Waterborne Unearthed Arcana.
The discussion on class uniqueness is an important one, especially with your goal at the end of talking about designing a class feature. I'm both of the opinion "only the amount of classes that we need" unlike the proliferation that occured in some earlier editions, and "make each class feel and play unique". I'd love to see you expand on that in future articles.
I'm not as strong a fan about the class archetypes. Conceptually, I feel you are trying to describe something like the strongly typed 4e roles, while I find the 5e classes to have a lot more room for customization in them. Cleric as a "support archetype" when you can have Light or Thunder that fulfill the "blasting and battlefield control" of what you describe as the mage archetype, Knowledge and Trickery for some of the "Rogue: The Skilled Archetype" or War for Warrior archetype. And plenty of other characters can fit into support. Paladin with their auras and lay on hands, bards with their healing/buff spells and inspiration dice to name a few. Really, 5e classes are open enough that trying to strongly type them by class can at best say "one typical build could be focused around...". I could build a druid in any of the four.
You address this briefly by saying that a class can be in more than one archetype, but considering most can cross lines not just for the class, but also depending on what build you want to go so a class can cross different archetypes for different characters makes these weakly typed at best. The background system, being divorced from classes, also has a large affect on this. A half-elf (+2 skills) with the Criminal background could easily hit the "Rogue/Skilled" archetype regardless of class.
I like that you are looking for the patterns between classes. High HD for "warrior" type is useful, but when you get something like the bladesinger wizard subclass from SCAG that grants powers around being a melee combatant but with d6 HD. Or even monks with d8.
All in all, the archetypes seem more like rough guidelines of what roles each class commonly fills, and even there there's so much variation I'm not sure how useful it is as classification.
Still, it's good to deconstruct and have viewpoints at what's underpinning character builds and I'm looking forward to further posts.
