I like the achievements idea, but the problem is we are limited not only to how many characters we can have in our sigs, but also to 2 images, so it would make awards a little harder.
One thing I was thinking of recently actually regarding ranks is inspired from another forums I'm on. Basically, as ranks increase, so do your privileges. In other words, as you get a higher rank, you are able to do things like put more images in posts, etc (I've just realised I haven't thought it through quite as well as I thought I had
). The other place uses a different software, but I would have thought that SMF would have some sort of feature that would allow a similar thing to be implemented.