I don't like arbitrary restrictions like this one. If it's my turn, I want to be able to complete it, not hand a carefully prepared board to my opponent. I'm okay with restricting the benefits of extra turns, like reducing mana gain or skull damage.
There is already one in place, I believe it's 1000.
Just no. If players don't want to stay within a guild, don't try to force them by threatening to limit their game experience. Too many sticks, not enough carrots.
Quite interestingly, your suggestion could easily be worded the other way around, prevent guilds from occupying an empty spot for 1 week. Which, using your words, will hopefully promote guilds to give members more reasons to stay. Just saying, I wouldn't like either side of the coin.
Rewards would obviously have to be scaled down significantly, so that the overall payout stays the same. I don't really see the benefits, is this supposed to strengthen the top guilds who have it easier attracting members?