One thing I've just considered - users who have a lot of power get more followers - because they have more power.

Yeah, there are a number of accounts like this. I try to deal with it by imposing a minimum cutoff for median reputation, but that's a somewhat arbitrary decision, and I don't know how to tell whether I'm getting the cutoff anywhere close to right. The overall follower score isn't really meant to be a measure of posting quality, just a measure of how many people are likely to see a post, so I think it generally makes sense to raise the score for a high enough number of low rep accounts (above some minimum).

Let me know if you have any suggestions for how to account for them.