We wouldn’t be surprised if the producers are already planning that far ahead. They know the popularity of the show and how much attention a sixth season would get, so they will definitely look into that possibility as much as possible before deciding whether the show should continue.
But if they can’t figure out any solid material for the next season, they should pause the production for the time being. Maybe not cancel it completely, because it’s always a good idea to leave the door open for possible future installments, but a temporary halting of new episodes could make sense.
In any case, season 5 is shaping up to be amazing so far from what we’re hearing from the various rumors that surround it, and we expect the writers of the show to truly give it their best with these new episodes. There have been hints that various fan favorite characters might be coming back for the new episodes, but there are too many of them to make any predictions.