In addition to the existing gamemodes, I suggest an option to play a random mode, which will place the player into a random Classic or Coven lobby depending on which mode they are on. This would likely not accomplish anything, but it could help revive some of the dead modes if enough people click the random button to make some of those modes playable.
To take this a step further, maybe the random mode button should have its own lobbies where all the players chose a random mode but are unaware of which mode they are in until the game starts. I'm not sure how this would work with Custom, but maybe it could wait until the lobby fills and then after the countdown tell everyone that it is a custom game and create a new period for the host to select roles and vote on repicks. This might have the issue of people leaving if they don't get the mode they like, but they can always sign up for the mode they want rather than joining the random lobby.