Clan War Chat With The Enemy
Basically a chat during the prep/battle period where you and the other clan (members in war only) can criticize, talk trash, compliment, etc.
It could prove helpful to encourage players to attack better after getting roasted for their 1 star, boost competitiveness, discourage rushed players, friendly clans would get a wake up call from the criticism that their teammates wouldn't give, hardcore competitive players would have people to talk smack to instead of their own teammates, encourage more maturity after a loss, etc.
Problems would be spam, harassment, people can be humiliated from warring again, etc.
Solutions would be the option for players to enable or disable red notifications from the chat and the option for players to mute an individual player.