Whenever I think about hypothetical PMAS sequels I imagine that Sabrina and co somehow managed to mind whammy Kyubey. Probably with some sort of block on forming new memories or something. So Kyubey still goes around contracting magical girls and "answering" questions but can't do complicated plots or anything and mostly sounds like a broken record. Sabrina and co would still be researching better solutions in the background but would be rather hesitant to do anything that disrupts the current one.
It allows for a steady trickle of magical girls being contracted and needing to be brought up to speed on on the real situation of the magical girl world. "What do do you mean, why aren't there any witches anymore?"
Plus it also seems like a relatively plausible solution to the Kyubey problem. Get a bunch of friends tougher with the right powers and make some sort of memetic anti Kyubey virus that transmits through their hive mind.