You could argue that Legendaries should know better. I think if we allow them a free pass on certain things then lower-ranked users would then complain that they're being unfairly punished. I think the only exception I'd be ok with is if they had their signature removed/banned and they lose their ability to earn via it here.
This. Take their signatures and avatars away and see how many of them are actually dedicated to the forum with no financial incentive. I'd guess this would be equivalent to permaban for 90% of them.
We could make a criteria 1 copy and paste per 1000 good posts. Applied to all.
This is unenforceable. Nobody in their right mind is going to spend time checking 1000 posts of some asshole to make sure they have only 1 plagiarised post.