Several post here argue the rules used in a current seniority based airline and apply those same rules to a non seniority based airline.
You can't, you have to use real world jobs that have established systems without seniority.
Pick any job that has a designated senior person in a dual or more role job. I guess an easy example would be a surgeon.
Surgeons would move around freely yet it doesn't mean junior surgeons never become senior does it? They through experience become senior. Saying an FO would never become a captain is simply not true because of so called leap frogging.
Abandon rank, we aren't in the military, simply be named or designated senior pilot and pilot, it puts an end to chasing command. Perhaps a course or some means would be used to show the surgeon or pilot has reached a level to lead the operation or perhaps the offer is given to be acting senior pilot several times. The more experience you have the more times you will be designated senior pilot.
The new surgeon/pilot to the hospital/company probably has a bedding in period before being set free too, perhaps they must fly to certain destinations prior to being approved to be designated senior pilot etc
The rest of the world functions perfectly well without seniority it's just a different system with different rules and we would need different rules and systems. Pay being an obvious one but again look to the current world systems for solutions.
20+ years - seniority based airline but I'm still with Ernest K. Gann when it comes to seniority.