This might be very difficult to implement, but it might be cool to have. In postal chess, there are so-called "conditional moves" that you can send. Essentially "if you go here, I go here" is sent with the move.

Maybe after you make a move, before you hit SEND, you can hit CONDITION. In so doing, you are allowed to make one move for the other side of the board, then hit a button, REPLY. At that point, it is your turn again, and you can make a move, then you hit SEND.

Your original move is sent, with no indication there is a conditional move attached. If your opponent makes the move you had specified with the condition, then your move is made instantly and automatically, and it is your opponent's turn to move again.

Just a thought.