I think I've mentioned that already, but again;
we should have multiple league ranks for same level, for example:
Career starts on level 1, rank B. When you finish in promotion zone, next season you play again level 1, but rank A where are other teams which got promoted from B rank. If you get promotion from level 1 rank A, then next season you would play level 2 rank B.
Advantage is that you don't need to change line-up every season because you play on same level at least 2 seasons.