|Mark Moreland Developer|
Arcadia needs a clean break from "humans, humans, everywhere". Why not a lizardfolk Empire, a theocracy of sentient tyrannosaurs or any number of other possibilities instead of " oh, look, more humans / [insert PC race here] ".
If there's a time to take a step into an alien culture that isn't dead set on turning everyone into slime-covered meat snacks (aboleths) or slave labor meat snacks (serpentfolk) but has their own distinct culture, Arcadia is the place.
Considering there's the lizardfolk nation of Droon in southern Garund, we are unlikely to also place one in Arcadia. That said, there are a ton of races that don't yet have a corner of the map to call their own, so it's entirely possible another "monstrous" race will get their own civilization in Arcadia as well as other places on the map.
As to whether Arcadia is the best fit for where alien, monster people live, that's a tricky question. One of the goals of the Pathfinder campaign setting is that there be representation for players of all sorts, and that includes many players who identify as native american (of any number of specific heritages). To insinuate, even unintentionally, that people from Arcadia (read "America") are monster people that you won't be able to easily identify with is less than ideal.
So expect something akin to Tian Xia with Arcadia and our other unexplored continents—many humans of varying ethnicities, as well as a smattering of other races, from elves and dwarves and gnomes to new ones, such as nagaji, wayangs, and samsarans were for the Dragon Kingdoms.