I am not sure how difficult this would be to implement, but I think it would be interesting to see on the forum main index a list at the bottom of the page of how many users are logged on. This could also show with a color offset or something similar, who the admins, mods, and senior staff are.
This would be different in scope to the Karma bar.
It would also show the highest ever number of users logged on this date at this time, etc.
This list could spread into the various main topic areas.
As someone who loves metrics, I think this would be cool to see and it is relatively common (I think).