I saw in other section proposal abt clan deathmatches. Inspired by that idea,however, I want to offer something different:
Alongside clan challenges and leagues, we could have clan sieges 9just like sieges for individual players).
Instead of sorted by timeframes - clan sieges would have standard timeframes, with constant number of players on each side (say 4 or 5 or 6).
They can be sorted by AVERAGE RATINGS OF TEAMS (like in Vikings Cup Challenges) - upto 1200, 1300, ... , upto 2000
There would be a queue of clan teams waiting to challenge the "board holder".
The only problem that I see is - at some point clan leader wouldn't want to play specific challenge that comes in a row (either because he suspects some cheating from his opponent, or because some of teammembers became unavailable or even left).
This can be solved by possibility to retire and not play the next challenge. This only makes it more difficult to hold the board for a long period.
Of course many details miss, but the idea itself can be implemented rather easily.