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The Nordic Countries (for multiple reasons)
1) Variation. Alola is obviously based on Hawaii and is a warm and tropical region, whereas a region based on Nordic Countries would be in cold climate. Also, if we go back in time a little bit more, Unova and Kalos are both based on large city areas whereas this new region would focus more on nature.
2) Giving Ice-types a chance to shine. A persistent problem with every single region so far has been that Ice-types are stuck near the very end of the game, which is why most players never have an Ice-type Pokemon in their team as they probably already have a full team by the time they encounter their first wild Ice-type Pokemon. A cold region would be the perfect opportunity to scatter Ice-type Pokemon all over the region, even at the very beginning.
3) The Nordic Countries are a perfect inspiration for a Pokemon region. You don't have to come up with anything because these countries already have everything you need for a good, varied region: large forests, lakes, mountains, volcanoes, glaciers, sea, archipelago and so on. This region would pretty much design itself and it would be the perfect playground for exploring in the wilderness.
Just fucking do it GF. Please.