Superclassing is a very beneficial concept to make the most out of reuse through inheritance. The Pega platform natively supports superclassing for data transforms. With a few changes, we can also use this functionality for activities. As a PoC, I've created my own component that adds superclassing to the activity ruleform - with only minor changes to the ruleform: 

1. 'Activity extensions' in the rule header of activities. The navigation icon works exactly the same, the toggle does not change the rules fired prior to the activity but instead, makes a change to the ruleform as seen in the next screenshot. This is due to not wanting to change / override final / internal rules from the platform

2. When the toggle is enabled, a new step is prepended to the activity that first calls the rule in the superclass (i.e. DMOrg-DMSample-Data-Circle -> DMOrg-DMSample-Data, and so forth for each subsequent activity that has this feature enabled)

3. Unchecking the toggle removes the step and removing the step unchecks the functionality. The same other 'rules' with regards to saving and refreshing apply.  I would love to hear some thoughts with regards to the usefulness of this feature for more than just sandboxing. It only entails a change in one OOTB rule and makes up for a small component of ten rules.

If anyone is interested, feel free to already download the RAP (.zip) of the component.