I really feel that an automatic multiplayer system should be included. Play by email is all well and good as an option but it would be much easier to have an automatic system.
Having all players connected at the same time with the ability for real time diplomacy or chat and a turn time limit. At the end of the time limit or when all players have confirmed their turns then the server host would initiate the new turn.
I don't think it would require a massive amount of change in how turns are saved and processed. Basically the sending to the host would be automatic instead of by email. The major difference would be the lobby style gameplay where each player must be connected and can see who else is connected and possibly chat to them.
I've only briefly skimmed over the DD for now so apologies if this is covered in there already.
" There's nothing like a bit of destiny to get the ball rolling " - Terry Pratchett - The Wyrd Sisters