In the USMC Warrant Officers who have served 8+ years as a WO/CWO can convert directly to Captain (O-3) if they so choose - they continue in their specialty but are "limited duty officers", which means they can never command squadrons, companies, etc. They can command non-combat units, such as logistics centers, admin units, training schools, etc.
They can be promoted to Lieutenant Colonel (O-5), but no higher.