Summary
MITRE is adding subtechniques to ATT&CK soon. It's in beta and soon will be in the major release. We should have integrations and update threat.* to add room for subtechniques.

Example of a subtechnique
https://attack.mitre.org/beta/techniques/T1548/003

Motivation:
Include any context around the suggestion and motivation for opening an issue.

Detailed Design:

Provide additional details around the design of the proposed changes.

• Field names: threat.subtechnique.{id, name, ref}
• Example values for the fields 003
• Suggested appropriate datatypes: same as the other threat fields
• Any example events that map to the proposed use case(s)
  No events yet, but here's a link
  https://attack.mitre.org/techniques/T1548/003/