There's already a highly-ranked Feature Request called "Allow more flexible recurring events" where many have been asking for this feature. You may want to cast your votes there, since it already has over 2,000 votes and is under review.
There's already a highly-ranked request called "Allow more flexible recurring events" that you may prefer to throw you votes and comments into. Many there are already asking for a "Recurring X days/weeks/months <from completion>" option. That Feature Request is already under review.
There's a new feature request started for "Recurring task based on completion," but I see many here are asking for the feature in this already-highly-ranked request. I want to add my enthusiasm for "Repeat X <from completion date> <from original due date>." There are rare cases when you actually want to repeat from due date (which causes an accumulation of overdue tasks if you don't complete them. Almost ALL everyday tasks are "repeat X days/weeks/months from completion." For example: mowing, washing the dog, watering plants, changing oil, giving pet medicine, updating finances, etc.