A couple of ideas that have been floating around in my head:
1. To attain ranks above Member, you'd have to earn some number of merit points. Merit points would be awarded in a monthly vote on best posts of the previous month, with various measures (TBD) to prevent gaming of the vote. Winning merit points might also come with a BTC prize.
This could work. I might suggest making receiving x merit points in a month be worth y activity, then increase the activity required to obtain each ranking.
2. Create or designate some sections as "serious discussion" sections, with no signatures. In those sections or maybe in different ones, also have poster restrictions such as Member rank or above only. And/or allow topic-creators to set these restrictions on their topics, similar to selfmod topics.
I am a little more hesitant to support something like this. While many people use signatures to earn income from advertisements, some people also use signatures to make political statements, support certain causes, etc., and I don't think it would be a good idea to prevent that in certain threads.
what other ideas do people have?
When an outsized percentage of users participating in a signature campaign are banned for post quality related reasons, the company behind the campaign should get called out by the forum. This is somewhat similar to what caused the PrimeDice signature campaign to close down, except that long standing users within the community were calling out the harm that the PD signature campaign was causing to the forum. This would give incentives to those ultimately behind signature campaigns to weed out low quality advertisers, as if they don't they will be known as someone contributing to the spam problem.
I would also suggest limiting the boards that contribute to "potential" activity. This would make it more difficult (and less profitable) to farm accounts with low quality posts.
On a similar note as above, I would suggest tweaking the "features" of signatures that each ranking can wear, being more restrictive to those who have lower rankings. We could also increase the amount of activity each person needs, while also increasing the number of ways someone can earn activity (see above for re merit points).
I do like the idea of a paid membership (copper) to allow users to "skip" the process of accumulating activity points to achieve a certain ranking status. If someone buys a copper membership, at the very least, it shows they have a genuine interest in participating in conversations.