I think I lean more to non-narritive sequels - unconnected sequels seems like it might be the best option, but as a GM I think it'd make my life easier if, at the brainstorming ideas stage, the simple caveat of 'we're setting it in the same country as a 1-10AP' was a thing. Think following up Quest for the Frozen Flame or Outlaws of Alkenstar doesn't need to follow on the story of the Broken Tusks or the themes of Outlaws. But it conceivably happening within a few weeks ride of the conclusion might make it a bit easier to have a 'you hear rumours that something big is happening to the east' as the plot hook between the two. I think this would also stop some of these regions feeling a bit one note.
After all Varisia features heavily in a full 7 adventure paths, and has ended up feeling like a well developed place where many things can and have happened. Cheliax/Ravounel is the setting for 4 adventures, and Kortos is in the same. I think the perfect example of how to do this are the 3 AP in a row runs for Varisia and Kortos - Rise of the Runelord, Curse of the Crimson Thrown and Second Darkness aren't sequels to each other narritively, and the tie-ins to each other are more easter eggs than actual connections. The same goes for Extinction Curse, Agents of Edgewatch and Abomination Vaults. So you definitely can have different, separate adventures with very little narrative overlap that are in places that the party that finished a 1-10 AP might concievably go to without being kidnapped by a teleporting wizard. Follow on Outlaws with a unique threat rising in the mana wastes, or Quest with an investigation of a demonic plot springing from the Mendev border or whatever.
The difference between that and an actual sequel is that it could also easily feature a fresh party (scholars from Nex in the Mana wastes, or Mendevian crusaders pursuing foes into the Realm of the Mammoth Lords), or be a sequel to any other 1-10 AP with the current method of 'somehow you get involved'. Admittedly it'd probably be a bit easier if the 1-10APs weren't pinging around the entire inner sea, but I think this might just be a product of the freedom of starting to do 1-10 APs to do the 'we've always wanted to write an adventure about X, but it wasn't a big enough idea for a full AP', and the first 3 1-10APs being written at a point where they were still putting out 1-20APs (at which point a party wanting a bigger adventure would probably just have bought a bigger adventure).