Email-to-activity processing – create Activity at future date
You could use the CiviRules extension (https://civicrm.org/extensions/civirules) for this. Every time an incoming email activity is created in CiviCRM civirules could create a follow up activity in the future. 

You need to configure CiviRules in such a way that the trigger is "activity is added", and that you have a condition on the incoming email activity type. And the the action is "create an activity" which is the "follow up activity".

Also note that civiRules triggers for each contact involved in an activity. The latter could be filtered out by a condition provided by civirules.

Documentation of civirules could be found at: https://docs.civicrm.org/civirules/en/latest/