The items for you to take this up a notch are the following: 1. Update things from time to time to make it newer-ish. The game does not feel like it has changed any of the times that we have played it over the four years (has it been that long)
2. Turn based for the die-hard workers.
3. Figure out the Iron Key for paying to play for a period of time. That really worked out for getting us to have one person pay for a key and then we could all play. If a person wants to get new friends to start a separate game, then you have an in for a new key purchase.
4. Ask your users, send an e-mail out to everybody and ask them what their issues are. Ask those that love it to post on their facebook / twitter / etc.