It was suggested I place this thread here. It is currently http://www.redhotpawn.com/board/showthread.php?threadid=42432
Don't know about the best format (duel, group, etc.), but how about each Clan with say more than 10 players, each have a Clan tournament. The winners of those tournaments play in a massive Clan tournament (or each Clan could nominate a player too).
If a player wins the right to represent more than one clan, then they must pick the clan and the other clans have the second place player represent them (or they just nominate a player).
This type of Tournament would take a long time to complete, so it oculd be like a yearly challenge.