As the leader of my clan, it would really be a whole lot more convenient whenever I start the war if there was a button or checkbox for "start war with previous members". This would allow the selection process to be virtually instantaneous.
Honestly a lot better than "flipping switches" and thinking about who to include. (A lot of players in my clan aren't that serious / that into wars so they don't even to bother to change their clan wars preferences.)
