Well, I suppose we could (mentally) go through a list of all the terrains and population types on Earth and see which ones we have had, and therefore predict future ones. The issues of the "plot" are a lot more complicated, but there are probably still some "classic" plots from many genres which can still be used.
Anyway, starting to think aloud (and I know we could argue about this or that place in this or that episode!):
ultra-high tech (Tollan, Nox, Nem, Asgard, Goa'uld/Tok'ra), high-tech ("New Ground", "Beneath the Surface", Orban), current-ish ("Other Side"), almost now but pre-space (not yet), steam (Vyus?), literate agricultural (Edora, "Scorched Earth", "Demons", Cimmeria?, Hanka?, Cartago), pre-literate subsistence/hunter-gather agricultural (Abydos, Simarka, "Spirits", "One False Step", Cimmeria?, "The First Ones", "Touchstone"?), Grecian style (Broca Light side, Chulak town, Argos), totally unknown (Reetou)
And locations/religions/styles:
Egyptian (Goa'uld/Tok'ra, Abydos), Grecian (Nem, Argos), American Indian ("Spirits"), other Earth historical ("Emancipation", "Crystal Skull"), woodland (Nox, "Broca Divide"), desert (Abydos, Vorash), tropical ("Touchstone), open rural (Edora), snow ("Solitudes", "Beneath the Surface"), water ("Watergate"), high mountain (not yet), marsh/fens (not yet), rain-forest (not yet), and so on.
Go on - what would be your favourite people/group/terrain to see in the future?