Tell me more about this subskill system idea.
I think
@AnotherGuy is talking about Skill Specialties. Level Up has this feature for it's skill system.
Skill Specialties (from Level Up's Adventurer's Guide)
In addition to having proficiency in a skill, a character may be an expert at a narrow area of specialization within that skill. For instance, a character proficient in the Deception skill maybe particularly adept in communicating through written code. A character gains two skill specialties at 1st level(plus bonus knowledge; see page 405), and gains an additional specialty whenever their proficiency bonus increases (at levels 5th, 9th, 13th, and 17th level). A character may choose any specialty in a skill in which they are proficient. A character may not gain the same skill specialty twice.
When a character makes an ability check to which their skill specialty applies, they gain an expertise die for that ability check. The Narrator determines whether the skill specialty applies.
Examples:
Arcana skill specialties: aberrations, constructs, detection, dragons, elementals, fey, forbidden knowledge, monstrosities, oozes, the planes.
Acrobatic skill specialties: : balancing, escape artistry, swinging, tumbling.