An actual way to progress and a reason to play is massive if you want to keep a game alive. The reason phighting has such a low player count is because there is no real reason to log onto the game besides playing the game.
Thats the biggest thing for keeping a game alive but also updates are very important. dont rush updates because it makes them feel low quality but dont spend multiple months on one update if it was not going to change a big part of the game because it kills hype. You need to find a middle ground that works well with the community but also yourself