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If they could develop PLA and SV in 3 years each, why would it be too early for another Legends game? That is, in November or January again if they really need the extra two months (outsourcing is a thing, so not really). The PLA engine has been waiting to be refined, not overhauled.
Anyway, they should make the game feel bigger than PLA, at least somewhat. Hisui had 5 biomes and 242 Pokemon, so surely Tohjo should hit 350-400 Pokemon and 6-7 biomes? At the same time, Johto is smaller than Hisui, so 4 biomes should suffice. That should leave room for at least two additional areas, and I'd pick eastern Kanto (from Pewter to Cinnabar) as well as Sinjoh north of Mt. Silver. The latter was stated to be a land in PLA, just like Kitakami.