They’ll have to make obtaining Alolan forms outside of Alola possible. Although here’s a question I have for you, do you think it’s possible that the Pikachu line is native to Alola? And that Raichu is meant to be a psychic type when it evolves, but without it’s Alolan Diet, ends up being a pure electric type instead?
I’m sure they’ll be obtainable in some way, yeah, same as the regular “Kantonian” morphs are obtainable in Sun and Moon. But that’s not at all the same thing as finding them in the wild with no explanation for how they got there when they’ve previously been specifically described as unique to Alola.
Anyway. Pikachu and Raichu. Unclear. The Pokédex tells us that diet triggers the manifestation of Raichu’s psychic abilities, but doesn’t really give us anything either way on which evolutionary path is the “original” one. Thinking in terms of Alola being an analogue to Hawai’i I’m inclined to see rodent-like Pokémon like Pikachu as introduced by humans, but potentially quite a long time ago – long enough to have adapted in surprising ways to their new environment. On the other hand, we know from direct empirical evidence that all Pikachu, regardless of their origin – Kantonian, Kalosian, everything in between – become psychic Raichu if they evolve in Alola (the only other Alolan form that works this way is Marowak, and that might literally be magic [EDIT: Also Exeggutor]). If we assume something resembling a real-world understanding of genetics and evolution, then that suggests that the psychic abilities are a dormant ancestral trait, present in all Pikachu but requiring some environmental stimulus to activate. Buuut it could also be that components of Pikachu’s Alolan diet – or even something else about Alola – are somehow mutagenic (or some mystical equivalent), and alter their genetics and powers in predictable ways. Or it could be that, as I’m inclined to think for Marowak, there’s no genetic component at all and instead there are Alolan traditions that allow them, basically, to learn magic (because, like Marowak, the Alolan Raichu form is associated closely with a particular cultural practice – namely, surfing). I actually had a massive argument with Jim the Editor over this one. He thinks that the Pokédex must be wrong about diet being a factor, since Pikachu can evolve into Alolan Raichu after spending literally minutes in Alola and without eating anything; I think that this is an edge case that doesn’t reflect the designers’ intent, and is a result of Pokémon’s mechanics for time and eating being extremely unrealistic. Also his interpretation doesn’t really suggest any other answer for why it happens other than Alola being surrounded by a magic field that gives Pikachu psychic powers for some reason. Basically he thinks that if the designers had meant for Pikachu’s diet to be a factor, they should have represented that with a change in the evolution method (he suggests an item called a Thunderstone Cake, or something similar).
So I’m a solid “maybe” on this one.