I put together a quick proof of concept that I call "recompletion" that I thought might be interesting for a few people here.
it adds "recompletion" settings to your course (similar to where you find the course completion settings) and then lets you schedule the deletion of activity and course completion data within the course. This seems to work in a large number of situations however is only at a proof of concept stage (not ready for production use) and has a number of shortcomings at the moment, for example, some activity types may autocomplete based on the data submitted within the activity.
If you're interested in taking a look I've published the code here:
You can see some of the existing shortcomings and ideas for future improvement here:
If you're interested in collaborating feel free to submit pull requests to the github repo or get in touch.