I am sure this will have been suggested before but why not have the option to play no time per move and say, 40 hours in the 'timebank' for 40 moves? In other words you simply have 40 hours on the clock at the start of a game or whatever time control is chosen. This would encourage plays to get on with it in the opening or suffer the consequences later in the game when more thinking time is required; as in OTB chess. It would only be an option with the present options remaining but I think it would be popular; I would certainly use it myself.
I'd support the idea of an option for zero time control plus 20 or 40 days timebank, but it needs to be able to cover the situation where opponents are, say, 4 hours separated in timezone. Otherwise player B could make a move at midnight and player A would use anything up to 20 hours before he even saw the game again. Since I don't think that's fair to player A, I think we may be stuck with 1-day minimum time controls for most games.