This would be an addition to GODOG's suggestion Previous Clans Window (with picture)
But what matters most to me is that NEW clan members should NOT be allowed to access the WAR menu for at least 48h after joining a new clan to prevent spying on trap placements and Clan Castle troops.
Or for new can members when they select the war menu they get a message "A war is in progress. Please wait until the war is over to access the war map" - this should be valid trough war preparation time and during the war

