Perhaps one option could be something similar to what has been introduced in Four-in-a-Row, where computer-controlled opponents can help fill the rooms when there aren't enough human players available.

I believe the idea there is simply to make it easier for players to find an opponent during quieter periods.  Something similar in FODBall could potentially help keep games moving and make the room more attractive to players again.

Of course, bringing back the activity levels of 2013/2014 is another challenge altogether, but making sure somebody entering the room can actually find a game would be a good start.