Give league managers the ability to merge divisions within their league at the end of a season and also to manually promote/ relegate individual managers, again at the end of a season.
This would help leagues struggling to get new inflows owing to minimum of 30 members in Division to appear to League search filters and manual promotion/relegation would help to stop managers from quitting the game. It is also a pretty normal feature in mobile games with alliances so there is no reason why it couldn’t be part of the league structure in this game.