Add a SKIP button
If three members are online, and the fourth is offline, let the three unanimously decide to skip a player's turn. frustrating when the last enemy is burning/bleeding and will die if the last player just passes their turn.
Maybe it sends an email to the player whose turn is being skipped, so they have the option to jump in if they can. They have 5 min or such to move. If not, the skip is granted.
Aaron Abenoja commented
add a vote and a timer for the one being voted out of the turn. is that a good idea?