Every season we see a new group of people introduced to the island.
S1 - Oceanic 815 survivors S2 - Tailies S3 - Others S4 - Freighters S5 - DI S6 - ?
I would venture to guess S6 would either feature a group unknown to us previously (e.g., Tailies or Freighters) or introduced earlier and revisited (e.g., Others or DI). Possiblities include the members and/or crew of the Black Rock, Hanso Foundation, Oceanic 815 survivors (again), Egyptians (ancient or otherwise), etc.
My guess is we either go back to the early past to the Black Rock to learn more about the island's origins including Jacob, Nemesis, and possibly others that date back to their time.