I think this is a great idea... and like you said it has sort of already naturally developed. I myself am guilty of multiple #milestone posts celebrating followers, number of posts, etc. If we built this into the platform, I think it would be a great incentive and also a goal for new people to set their eyes on.

HOWEVER: Lately, I have seen a lot of people just go crazy about getting followers. There is one person I follow that got to 1000 followers in less than a month. Yet, his posts still hardly get any feedback and his content is not necessarily worthy of the crazy number of followers he has. I think for some people, the incentive to get 1000's of followers is greater than the incentive to really build a meaningful Steemian community. I have been on Steemit for 4 months and only have 220 followers even though I am very active... yet by the standards of some people today I would seem like a super small minnow because it took me so long to get those 200 followers. IMHO there is a disconnect between number of meaningful followers and number of total followers, so gamifying this aspect of the platform may be detrimental to some.

Just my opinion though. Cheers and keep up the good work man!