People like to visit active and busy boards, so I envisage this add-on displaying a random number of members (perhaps a percentage of your total membership) and a similar random number of guests (perhaps member-percentage-total * 3 or somesuch other formula) so that on the homepage people see that other people are active on the board.
I don't think it needs any complex algorithms for dealing with times of the day etc. just enabling your forum to look busier than it actually is.
Maybe a setting for the percentage to "simulate" so that as your board gets naturally busier you can turn the pseudo activity down.
Would this be a difficult add-on to make?