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What about a favorite poster plugin. - any interest here?
You could click on up to x number of posters and they would be "Favorited". (perhaps limiting number to 5 or 10 favorite posters per individual).
And in their profile it would show a list of people who favorited them and maybe a scoreboard for the 10 most favorited posters in the forum.
if there is interest (and perhaps a few pledges), let me know, and I could begin another plugin project 
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I may not provide the completed solution you might desire, but I do try to provide honest suggestions to help you solve your issue.
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Post ratings/voting with stars or thumbs and sorting posts by votes with ranking number, like most votes goes to top nr1
I think this would be a cool activity type filter. Only show activity from your favorite users.
Good idea.
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In parallel to "Favorites" it could be called "Friends" or "Follow". An icon similar to the "Favorites" star could appear on the profile page of the user floating on right side in line with the user name (like next to the [Message] button in this forums profile pages).
I think it's a cool idea, if possible i wouldn't limit the users you can follow or at least make it not below 40.
Another question: Would it need a setting to turn this function off if someone doesn't wan't be followed? (privacy thingy to make it not so easy to get stalked).