(Focusing on the functional side: I would not wish for a moment to discourage you from getting into the fun and reward from writing a plugin.)
I’ve done this manually in the past and I think it’s a great way to highlight positive contribution.
The Top Topic List already gives one a really good guide as to which Topics moved and shaked the community in the last period?
One solution might be to simply pick a Topic from Top and edit the OP with some kind of additional heading and then pin it? Choose the Topic within a short chat within the staff group based on a shortlist from Top?
Another thing to consider is the fact that the top contribution might actually be a reply (ie a Post). Perhaps you could link the top Post from a monthly regular Post of the month Topic?