Remove one preseason game, add a second bye week, that will generate more income than the 4th preseason game.
If you want to remove a second preseason game then add teams to the playoffs. I also think instead of a preseason game they could have some kind of fan appreciation event/pep rally at all 32 stadiums, make it a TV event where they bounce around from stadium to stadium. Let the players speak to the fans, talk trash about the Pats etc.
18 games sucks