You can make a suggestion about it, though I don't recall any other mentions of this sort of thing. Note that the members online widget is not limited to people online in the forums section -- it's anyone using any portion of your XF install (so help pages, registering, search, member profiles, etc). I can't really see the context/meaning of that widget changing, even if we did add more specific tracking.
The closest existing suggestion that I can think of relates to people wanting online counts for individual forums.