what will be criteria for a map being allowed as a league map? will there be limmitations? and what will decide what map is played for a particular match?

It will need atleast one block spawner and block deleter, both flags will need to be capturable without any building, adequate spawns and spacing so players don't get telefragged (fixed in 1.9.1), and anything after that is just up to the discretion of league admins I think. They'll decide whether a map is up to par or not quality wise.