A proper Clan War system. Where you actually play against the player in other clan you have war against. Could be a singke Bo3 battle only through all 4 battle days. So that there is sufficient time to play your battle.
Could add between-clans chat with auto-translation for better coordination
Or, give every player an ability to be able to book a fight. Like, I will indicate that I will play at 13:00 UTC on Saturday, and someone from other clan can book a clan war fight.
Or, just a button that “I am ready for battle” so that enemy clan can see that there is a fight available.
If there are no players online in an enemy clan, only then allow random matchmaking in clan war, but with a warning messege that informs player that it is a random opponent