Clan leaders should be allowed to choose who in their clan is going to fight.
If someone in a war drops from the guild during prep day a sub should be allowed to take his/her place so the attacks are not lost.
Additionally, if a player requires a week off when going on vacation, but doesn't want to drop down from a high trophy count they should have a holiday option that lets the clan leader know they will not be there and the clan leader can remove and insert someone in their place during war prep.
From what I've read, Supercell isn't too high on an option to be excluded from war (leader or member). They feel it will lead to bullying, "uncheck your ready to war button or I will kick you" type of thing. I don't agree with that assessment but that's what they've said. I'm sure it would happen in some clans but not good ones.
I do like the idea of a war substitute. One, maybe two, members who were left out of the war that only get to come into the war if someone leaves on the preparation day. Once battle day starts there could be no changes though.