Just this on its own is an awesome idea.
there are always tonnes of 'how to stop hopper' threads but this is the first time that i've seen that suggested.
theres no incentive to hop if you cant take your cc troops with you... hopping over!
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I think there are two kind of players leaving a clan during war.
The ones just heading out for a good reason to return back afterwards. For example, our last war I had to attack enemy's number 1, and I really needed a few really high troops in my castle to 3-star the base. I went out to a clan I know with some very high level players, got me the troops I needed, returned back, done the attack, 3-starred it and we won the war. If I had 2-starred it the war would have been a tie, if I had 1-starred it we lost.
Why would anyone want to force those kind of members to not leave the clan?
The other kind are the ones who leave because they no longer like the clan and no longer want to participate in war. Those players are already demotivated to stay in your clan and forcing them to stay will demotivate them even more. That will probably have a negative impact on your war results.
Now, I do understand the issue as I have faced it as well. The only solution I can think off is that clanleaders can put lifetime bans to players, like a clan blacklist.