I think it wouldn’t be so far off to have all classes free for all races. I mean, even in game we got extremely good examples for these scenarios where a race and a class mixes that is not available for players:
Khitai + Barbarian = Hyrkanian Raider, I personallly think playing as a Hyrkanian Raider would be really cool, it already kind of works as ranger but the Hyrkanians themself got these pure dual wield barbarians among them in the game.
Cimmerian + Necromancer = Outcast cimmerian witch / witcher, I think this would be a plausible scenario too, there will probably be some people in Cimmeria that get drawn to dark magic and try to grasp these forces but for being like that they get pushed out of their clans.
Stygian + Dark Templar / Guardian = Those are probably the most possible ones, just look at all the guardian type enemies in stygia in game, a scociety that relies heavily on nobles will most likely have well geared strong guards, Dark Templars could very highly be some kind of “body guard” of Thoth Amon. Also, look at all the guardian-like enemies in Turan, General Arman is a pure polearm guard and given that Turan got a huge army in the lore guards / soldiers from these kinds of lands would be really appropriate.
The only thing that could make “problems” are the priests. The fanatic Tempests of Set probably don’t have foreigners in their rows since they most likely all come from the upper / noble classes where foreign people like Aquilonians or Cimmerians could never be. Stygian Priests of Mitra could work if it’s some kind of convertite maybe. Bear Shamans on the other hand could totally work as Aquilonians, Zelata could be a good example as she is supposed to be a shaman or witch as far as I know. Stygians might be a bit problematic as Bear Shamans but then again, what about all the “savages” in the Red Lotus Swamps and in the Zaara Oasis? A Bear Shaman (or maybe some other kind of animal shaman) could totally work out for these. Jhebbal Sag is called “Lord of Beasts” and is worshipped in the Black Kingdoms, just as an example.
Sorry, too much writing, I just love character customization when it comes to appereance, clothes, weapons and so on