Very simple idea. Each alliance has a war footing, which is set by the leader.
The first one is what all alliances have now, where every player’s participation in wars is set and forget. If you’re in one week, you’ll automatically be in the following week unless you actually change it. Ditto for non-participation.
The other optional footing defaults everyone to non-participation every week, and every player has to do something to opt in and confirm defense team, exactly like it is for raid tournaments.
This would solve so much misery for players in more casual alliances who, week in and week out see one (or several players) “participating” in wars but not using any flags.
A further step to this would be to have another optional setting where war loot rewards is influenced by flag usage. The more flags you use, the better your chances of getting better rewards. Again, this is an optional setting to the current.