Another must, especially while the project has low traffic: allow challenges/accepts and plays without both players having to be logged on at the same time. The high scores list shows that only four people have actually completed one or more games.
I hope you can regain an interest in this site or at least post the source code somewhere. The world needs a better rps server, and this can be the one.
Even though it's been a year without a reply, don't give up! I love the simplicity and minimalism of the site. My suggestion: focus challenges on a certain number of repeated games; minimum and maximum preferences and an automatch feature would be useful. Without repeated games with the same person, where's the long-term strategy?