Matching market to league level is a decent solution I think. Problem is the difference between previous season income and new higher level cost - but won't worry higher level players.

Bot 'filler' teams could work but there are risks that higher lvel teams get surrounded by bots so would need to be managed.

Server Management could help by merging to keep populations up but publishers don't like merges as it sends out the wrong signals and provokes 'Game dying' rants.

High Level Clustering is also a possibility by pulling teams above a certain level onto separate 'higher level' servers.

Ideal solution? A mix of all the above.