Personally I'd like to see The amount of steem power of the followers to be factored in. I've seen serial spammers with 200+ followers as they tend to follow each other. From my experience, Average comment length and reply length is also a very good indicator of a productive steemian. I'm not a professional programmer. So I don't know how implementation would work.

There are multiple ways steemians produce quality on the platform. So personally I think the follower count, comment and reply length, average post value, Average comments per page and steem power are all good indicators. So it's wise to incorporate them all.

Also I'd like you to check out @miti's effort to combat spam. You can read it here: https://steemit.com/steemit/@miti/my-commitment-to-making-steemit-a-place-free-from-spammers
He/she is fighting the good fight for steemit. I have no affiliation with him/her. I just came across the account and it's really something I've wanted to see after my almost effect-less attempts to flag serial spammers with my minnow account.

Please go check it out.