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I reckon there won't be as many new Water types, there's already plenty of Water Pokemon they can put in Alola, depending on how many new Pokemon we get, I can see there being 3-4 lines, something land based, the others ocean based.
Gamefreak should really try to balance out the amount of types each generation has, I mean Kalos only had Diggersby as it's Ground type (not counting Zygarde since it's a legendary) and that's not much of an option.
They should definitely try get at least 3 lines of each type.