I think [...] should allow [...] signature (campaign) only for xx ranking
I think this would only drive up the price of these ranking accounts, and drive down the price of accounts. I don't think this would do very much in terms of reducing signature spam, nor would it do very much to address the issue of low quality posts.
You can't know that. You're just speculating about potential cons. Every single solution to this problem is going to have cons and there are going to be people complaining because they're losing their opportunity to spam and earn money by breaking the rules. I'd rather we try-out some solutions that seem good enough, than infinitely discuss how to tackle this problem.
To post this kind of stuff?
-snip-
I think a general thread used to report spammers (& copy-paste spammers) wouldn't be a bad idea. We've had a lot of these threads occasionally pop-up (sometimes singular and sometimes bulk reports). Since this is already happening, it wouldn't be bad to streamline it into a singular one. I'll talk to theymos.