So I have put the Wizard/Witch example below forward in another thread, but I figured I'd give this its own thread and expand on it. I really feel that a lot of the non-Core classes would work just fine as archetypes, especially now that we know they really are gunning for more customizeable classes. You just have to build the core classes right to begin with.
Let's start with the Wizard and Witch, since that's where I first formulated this. If you build the Wizard right to begin with, it becomes a great framework for archetyping into the Witch or several other things. So, we all know how bland and underwhelming the specializations have always been, right? Well, let's fix that.
Every 2-3 levels, your Wizard gets to pick a Specialization Knack from a pool, just like Rogue Talents or Rage Powers. Not only does this suddenly mean that not every Abjurer is the same nor every Necromancer is the same, because you get to customize, it also means that the general quality of these abilities will increase a bit as well due to having to compete with other options available at the same level. I'd expect that just like Rogue Talents, at high levels you start having the option to pick from a better pool; or alternately just have one big pool with various tiers breaking off at level prerequisites. Your specialization then presumably gets a capstone at 19th or 20th level.
Okay, so now the Witch. The Witch is suddenly viable as just an archetype that replaces "school specialization" with "witchiness." Instead of picking from school-themed powers, you are choosing Hexes. Your familiar is now your spellbook, but otherwise doesn't really require a lot of changes. You still get a capstone at higher levels in place of the specialty capstone. Just like a specialty school you lose access to a couple schools of spells, but these are chosen thematically (probably Evocation and Abjuration) rather than by the player as with a specialization.
This basically works for the Summoner too. Instead of specialist knacks or hexes or a familiar, what the Summoner gets is their Eidolon which is built up and customized over time. Instead of bonus spells per day in their specialty school, the Summoner can spontaneously cast Summon Whatever increasing with the spell level they have access to. Certain abilities of the Summoner such as Maker's Call and Transposition, which maybe should just be combined into one ability since they're so close in level anyway and use the same pool of "uses per day," would just take the place of one of the Wizard's bonus "Wizard feats" at the level where that feat would otherwise be gained.
This same thought process works just as well for every other class. Build each class so it has a "path" on top of its bonus feats, that confers abilities within the path as the character levels up on top of the more general "class bonus feats." For example, the Fighter could easily inherit some of the path options given to the Soldier in Starfinder. Now, an easy option for an "extensive" or "total package" Archetype that runs from 1-20 is to replace the path selection of the class in question. With our Fighter example, instead of being an Armor Storm fighter (using an SF Soldier example), it can now be a Cavalier.
I hope the devs and others see the potential in building classes out this way and the options it opens up for archetypes. :)