And if they did, they don't have to obey any other "rules" either. It could be sequels in the sense of GS, just with the setting still being Hoenn, meaning new pokemon popping up and a dozen new locations.
Im curious whether they would do something interesting with that small icy cave in Shoal Cave if they took that route.
South of Route 118 on that close unused island.
Northeast of Petalburg
South of Fallarbor
North of Route 121
East of Route 123
Or along any existing route if they wanted.
Those would be potential places looking at the GBA map. However as the map would be bigger, they could put it anywhere that's not in the middle of the ocean.
To make sequel they need one big space for some new cities and routes next to each other, not small areas you mentioned. Anyway, they can't put new cities/towns in every unused area because there has to be some space between locations to make them separated from each other. That's why every region have unused areas, Unova still has them even after extension in B2/W2. It's far more likely that Hoenn was made with remakes in mind, not sequels.
Before HG/SS, Johto looked like it could have new cities because of huge land connection on the west, but no cities were added, only cave entrance, two routes, Safari Zone and Battle Frontier. If they refused to make new cities on the west of Johto, I highly doubt if there will be any new city in Hoenn.