Reminders: repeat reminder based on date last completed
I’ve been waiting for this feature for years, and Apple never adds it. Useful for so many things: If I want to get a haircut every 6 weeks, but I’m late getting it by a week or two when I’m busy, I still want to wait 6 weeks before I get another haircut after that. 

The point of Reminders is that I can record them and stop thinking about them. If I have to say to myself, “oh, it’s not time yet for a haircut because I just got one 4 weeks ago”, that’s having to remember it.  Or if have to adjust the Reminder, that’s more effort than I should have to put into it.

So I’ve been using GoodTask for about 2 years now, simply because it offers “Repeat after completion”.