Currently theres a 4 man up to pro and 4 man up to elite assuming above 16 membership in a league as standard but most leagues struggle with both maintaining a strong pro league and also sometimes keeping those in pro interested with strong competion since that is mostly just found in Elite.
This usually results in of the 4 people promoting to pro from rookie maybe 1 maybe 3 will become inactive either before their promotion to pro though will still have the points so will promote into pro regardless or during pro This means that eventually these inactive will be kicked by the league host or automatically after many days by the automatic system, so of the 4 maybe 2 are actually contributing to bump the number to 16 but since the promotion to elite is the exact same number it means Pro is forever in a state of flipping around 15 member and promotions being turned on and off. The only exception to this are extremely strong leagues where they have an Elite demotion but generally being demoted from Elite is a sign of inactivity and if not I'd assume the member will just lose motivation in the league and stop playing that league or the game.
The possible solution is to increase the number of promotions from Rookie to pro while maintaining the pro to Elite at 4 so Pro has a input then its output. Obviously to counter this it would mean that should pro be too large the deomotions would similarly be larger but thats not a problem. Whether this is doubled to 8 or even just to 6 it would mean that pro has a greater chance of carry multiple interested players into pro and should some leave like is nautral there was a stronger influx then like is the case currently to make that tier fun. Eventually this should hold player interest into elite making the league as a whole at all levels more healthy.