How about this?
Give the users an option to chose how long they want their season to be.
Make it a setting in the game......Default to 4 weeks (cause I prefer it )
You are stuck with your choice for 12 weeks (6 2-week seasons....3 4-week seasons....2 6-week seasons)
Arrange players in leagues acc to their preferences.
Simple solution.....should'nt be too difficult to implement.