FOR BASEBALL, September is the time when the crowds thin in most parks as children go back to school and the summer fans return to work and football. The regulars still turn out each night to watch games being launched into coral autumn sunsets, feeling a mixture of melancholy at the approaching end of it all and gratitude for the elbow room in the stands. In a few fortunate cities where the pennant race is being run, the excitement of summer continues undiminished. And on quiet September nights after the stadium lights are extinguished and the crowds have dispersed, you may hear the still small voices of network executives raised in prayer to their secular god: "O, Nielsen, let it be Los Angeles; let it be New York."

This year their prayers may be answered. In both leagues the teams from New York and Los Angeles stand good chances of winning their division championships. It isn't too farfetched to envision the playoffs and World Series being played out entirely within the confines of those two great media markets -- which as far as TV ratings are concerned, would be like hitting the Lotto number.

It's still a long shot, of course. Teams from Toronto, St. Louis, Kansas City, Cincinnati stand in the way. But it would be fitting if this bicoastal fantasy were to be realized in 1985, because this is a year when baseball has been, even more than usual, preoccupied with the subject of getting and spending.

For one thing, there was the strike -- however brief -- as a reminder of the stakes involved. The games themselves -- those on television, anyway -- were lengthened a little by providing more time for commercials between innings. The commissioner has promoted an effort to make baseball even more of a family affair, and to this end the gentrification of crowds has proceeded apace; ther is a growing number of pastel polo shirts in the bleachers, which makes them not quite the bleachers anymore.

Meanwhile, the season keeps expanding to provide TV dates. The regular season will end this year at about the time the World Series used to be reaching its climax. The playoffs are being enlarged from a maximum of five games to seven. The network televising the World Series ordered that all games be played at night, which raised the issue of lights at Chicago's Wrigley Field. The Cubs resolved it by falling more than a dozen games behind.

With a bad run of rain delays the World Series could be concluded in early November this year. Most trees will be bare and some people may be sitting home watching it before a roaring fire. Whether it's played in Los Angeles, New York or Toronto, the commissioner, the league presidents and the network executives should be required to attend coatless and in short-sleeve shirts, and to smile heartily the whole time, including the extra interval for commercials between innings.