Just a suggestion, daydream, but maybe the promotions/relegations should be done only after the first season, and in the first season we sort out who will be in what league.
What I mean, is that whoever finishes 1st or 2nd in any of the leagues goes into League 1, whoever 3rd or 4th in each league goes to League 2, etc. Would mean that, generally, players are ranked according to their skill, rather than their time of joining.
added the new joiners AND added keylarions suggestion, which i like a lot. i was wondering about how to make this interesting for people joining late, because starting out in the last season is understandably not quite as interesting (although more challenging to make it to the top then...)