How about a mod for a forums digest (selectable on/off by the administrator for each forum). It would allow members to choose to be sent a "digest" of forum topics every so often (Admin could set it on or off in the CP, then select whether it is weekly, monthly, etc. or a number of days between digests). If a member selects a forum digest to be sent, then the sytem would send them all the new posts or posts with new replys since the last digest was sent. The digest would be in the forum of an email with links to all the new forum topics or replys, etc.
Just a thought