Something along the lines of using their dated post count in combination with a like ratio to automatically determine the member each week and month respectively, week to week, month to month, not taking into account their history beyond these periods.
Also maybe a page in XF members tab that shows the last x member of the week, and x member of the month, then any residual data is dumped, as I don't need any growing DB table or such getting out of control. Unsure how the best way to do it is, though for a large forum, so overhead has to be minimal, certainly nothing more than a single query.
The actual display in both CTA and forum home sidebar... something simple and elegant, their avatar, username and maybe their posts and likes in that period only that earned them the title for the period.
How much? Whether it be custom just for me, or less if you want to release it and on sell to others, just let me know.
It MUST use XF conventions and best practices for Xenforo.