I have a next idea to make a multiplayer at official server paid by subscription.
Main game must be free, singleplayer is free and free download, source code also is free, and game at the local network with friends also must be free, but if you want to play with random players all over the world at officical server, you must pay your money, for example - 5$ per month. And developers with this way can earn as much money as audiences can attract.