IPB really could use a system showing how many people are viewing each forum from the board index page similar to how a lot of vbulletin sites have. There have been several mods for this, but not a single one worked properly. The current one doesn't work in certain forums for some weird reason. Another works, but does not add up subforums in the number.

This is probably the main thing I would like as a feature at some point.

Couldn't this amount in more queries?

Well one of the mods did it as a task each minute. One did seem to add a query per forum. But one or two other versions didn't seem to add more than 1 total query, if I am not mistaken.