If I start a Master League I've heard that I can't play in a singular league (like the Premiereship),
You can create the Premiership, but it has to have 16 teams in it, and yes you have to unlock the "league editor" to do this.
I've just completed my first season in the Master League, I never bothered with the Master Leagues in the previous versions but tried this one out, and it's really addictive.
There are four "top" leagues, and one division 2.
*League A (16 teams)
*League B (16 teams)
*League C (16 teams)
*League D (16 teams)
Division 2 (8 teams)
If you don't unlock the league editor in the PES Shop then you will start in Division 2, which has eight teams. Also the leagues are random. So like Mancester United could be in AC Milan's league.
The only thing you do choose from the start is what top league you will get promoted to. Then the teams that finish in the top two from your division will always get promoted to that league, and from that league two teams will get relegated into Division 2. The other three leagues will just act as foreign leagues on their own.
I think it's 2000 PES points to unlock the league editor (it's called Master League tuning in the game

, which isn't a lot really.
What I did when I got the league editor was rename the leagues
*League A (16 teams) = Serie A
*League B (16 teams) = Premiership
*League C (16 teams) = La Liga
*League D (16 teams) = Eridiversie (Dutch league)
Obviously for each league I just picked the best 16 clubs from each country, i.e Crystal Palace, West Brom, Portsmouth (don't like them) and Norwich failed to get in the Prem.
I then picked to go Sampdoria and chose the three worst Italian clubs, and four worst Spanish clubs to go into Division 2 and picked League A (Serie A) as the promotion league.
It's realistic to tell you the truth, I'm normally one for realism.
First season on ***** stars difficulty level, I've won the Division 2 and the Division 2 Cup. I'm in pre-season now. training my players

It kind of reminds of of Sensible World Of Soccer on the Amiga, where you could buy and sell players.
You can qualify for the European Cup/Champions League, I think it's called WEFA Cup. But I haven't obviously done that yet so you'll have to ask someone else for that.
You can play in the Premiership with 20 teams but not in the master league.
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I hope someone doesn't come along and say "you can have 20 teams in each league". Or that will p1ss me off

. But I've read the manuel and it seems that it's just limited to 16.