I joined 2 mediteranean maps and on both my ally want inactive, as did the enemy. Probably the map is too complicated for these >self-censored bad word< players.
Hence, I suggest to restrict the map to a certain rank. This at least gives you players that have played a few games and maybe... just maybe... will stay active.
More experienced players will also at least have some clue on how to use the wealth of units they are provided with.
It definitely is a great map though. I love it how the navy and airplanes are more important (even crucial) for dominating the sea, while you still have to fight your regular land battles too. This adds more spice and variety to the game.
Player Level says nothing about its activity. I have the highest level, but even I slid 3 fights. I know the players in the top 10 that come into play, attack, get points for the attack and send the map to the archive.
The best way to determine the proportion of active players "Number of Points / Number of games." Skill players can be determined by the proportions of "trapped provincial / lost provinces." You can offer developers an idea to make a simple script that will multiply these two parameters, and create a separate rating. The first top-300 get privileges such as access to the Mediterranean map.