Which race would you add to Heroes?

That's also possible. It's just that the concept of a "core" race feels a bit unusual. What would be the "core" race for the Barbarians in the first or second game? Orcs? Trolls? Humans? Wolves? Having a "core" race isn't really a necessity. You could create a Warlock castle with black mages at its head and their mercenaries, like thieves, minotaurs, and others.