For example, one idea I had was to allow users to subscribe to campaigns that other users set up, and then the campaigner could automatically push signature updates to everyone subscribed, and also track exactly when and for how long each user was subscribed. Would this be significantly useful? I'm not all that familiar with how these signature campaigns work, so I'm not sure.
I can imagine there's a use case for this, if a campaign manager replaces the one short-term signature with the next one, while keeping the same participants.
However, I wouldn't want this: I want to be in charge of what my signature shows, and I wouldn't advertise anything I don't believe in.
This may be useful for a few (lazy) campaign managers, but I don't think it will do anything good for the forum.
I think enforcing
Signature Campaign Guidelines will do the forum more good.
The only other suggestion I can think of, is something I've suggested before: only allow signature campaigns that pay in Bitcoin. The ones paying in made-up tokens have no real cost for the ICO, and thus don't mind "paying" for spam. If the campaign pays in Bitcoin, at least they have something to lose.