it isn't nice, that some clans have problems to get a challange against another clan, because the team is much stronger or much weaker than the other one.
There must be a rule that every clan have the chance to get 1 challange every month with 100%, also when the other clan don't like it.
It could be managed with points for example. Every clan have 240 points the year - a challange cost 1 point for everey member in this challange. Every clan can only challanged in this way by another clan once the year.
The clanleader pays with the offer his points.
An additional rule could be a defense against such challange, this could cost the same on points, from his own pool.