Case Dormant for x Days Alert

I've already submitted this to support who were kind enough to forward to Dev team.

Essentially I want to send an alert if a case is NOT updated for X amount of days. Just to make sure it's not forgotten!

I thought time based rules would allow this but a rule can only run on creation, update or edit so this negates the point of using the last modified time field.

This would be s great feature for cases but also opportunities, leads and accounts.

Any thoughts appreciated...