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They've been changing up the formula for the 3rd versions, making them more inticing to buy rather than being just a simple retread of the vanilla versions.
Alola has a disgracefully low number of new Pokemon and Alola forms. We got more Megas in X/Y than we got Alola forms in S/M.
Hawaii is also compromised of 8 islands, not 4. My guess is they'll be another 4 islands to explore come stars, with new Pokemon and Alola forms. Maybe not the same size as the already established islands, but pretty meaty nontheless.
But how will backwards compatibility work with S/M if we're getting new Mon? It won't. Well, we won't be able to trade back at least. It'll be like ORAS and X/Y with the new megas. Unless they decide to patch it which is always an option.