It's too big a leap to assume they work on an expansion but aren't letting on yet.
I think they might have painted themselves into a corner with some of their innovations.
I'd very much like to think that something is in the works, that isn't more living story, but an expansion? Above all I'd truly love some fresh, and challenging, dungeons. Some properly difficult end game maps, that need groups. I can't see an expansion on the horizon for many reasons though.
An expansion is often ten more levels, maybe a new class, some maps and perhaps a new raid and dungeons. They'll mix a few things up - stat rework, new skills, new armours, new grind. The old content often becomes near worthless; something to quickly pass through on the way to level cap.
They didn't want vertical progression. There was enough uproar over ascended. So I don't expect more levels, or armour tiers, or major stat reworks.
You don't need to grind your way through a particular zone to get to level cap. You can, but you can also wvw, craft and gather, or repeatedly do events in Queensdale. So any new areas are going to be optional, or appealing because of new unique skins, not because they are essential for your level. With level scaling anywhere in the world can get you to end game. You never need to set foot in Orr, or Frostgorge. You need never leave the Halloween labyrinth!
The story (main quest) isn't used as the route map to zones and quest hubs from which you endlessly kill another 12 boars.
So where does an expansion fit in those terms?
I question the focus on temporary content. It is not repeatable in the same way new hearts and maps, races or dungeons would be. I think there needs to be more of a balance between the two, with some new dungeons, raids, and other repeatable content as well.
We can't even repeat a lot of the great content they have made - marionette, molten facility etc. It's been made, used, and thrown away.
So I don't think they can tell you that they're working on an expansion, because I don't think they are.
Is there a market for this way? Maybe, but perhaps some of the more game centred people, and I, will be lost along the way. Perhaps that's some of the rationale for the simplification of the New Player Experience etc.
If right, I'm sure going to miss GW2 combat.
Hopefully ANet can come along and prove me very wrong in the New Year.