Some classes have something akin to sub-classes. Rogues have Rackets. Druids have Orders. Clerics have their gods. Wizards have Schools. These additional class feature choices do anything from giving you different bonuses and proficiences, like in 5e, to offering up different starting spells, like with Wizard schools.

But yeah, most of the focus, both from the core designers, and 3rd parties, is on archetypes for creating different permutations of a given class. People don't like to touch the underlying class chassis.

Rules exist around characters crafting magical tattoos in the game (found in Secrets of Magic, Pg. 164), but there doesn't seem to be much guidance for GMs trying to design their own.

Most of the tattoos that have been published have active effects that can only be used once per day, in addition to a passive boost to some skill.

I assume you have a wider vision for this that could necessitate an actual archetype, but just based on what you've shared here, I think you can get what you want just by designing some tattoos that give passive improvements to AC when the bearer is unarmored. If you want to go the full archetype route, you can specify that the tattoos only work on characters with that archetype dedication.