I'm not sure if it has been mentionned already, but how about a Clan open invites feature? A leader could place an invite for a challenge for x members, with an average rating and time frame...
I think about that because I'm frustrated of sending challenges without receiving an answer. The leader is online (or he was recently), and nothing happens... I mean: Accept it, bounce it or delete it, or pm me if you want to wait a bit, but DO SOMETHING!!!!
If I go to the open invites, I will be able to accept a challenge RIGHT NOW.