It would be great if there was a feature to either reopen or remind the assigned agent to do something on a ticket. This would be something that is set each time and not a rule that could run and remind someone.
And would suggest that this is not at a workflow level. Perhaps something at a ticket level to remind the agent to get back in touch on 'this date'.
Actually - you can already do this with tasks. We don't tend to use tasks across different groups (or are trying not to) due to visibility issues of tickets. So tasks are great to give yourself a reminder. Just did one now to remind me to get back in touch with a customer next week.