Other teams offer you the chance to manage them in the master league
This feature used to be in a few older games such as Sensible Soccer.
After a few seasons if you were doing well, other teams would offer you a contract to manage them at the end of a season.
Adding this would make the master league much more interesting and add months to the playing time as once you build a team that keeps winning everything season after seasons you either stop playing or start again.
Its also interesting starting at another club and playing against the team you built