Completely disappeared without a trace. And deserters, who should have been brought back from previous installments of Heroes.

I think the "yin-yang" principle of opposition should also apply to the heroes.
For example:
Humans-Demons,
Elves-Dark Elves,
Mages-Undead,
Dwarves-Barbarians (coming soon),

Humans-Demons is a somewhat inaccurate division... many see the opposition as Humans-Undead (living-undead)... but in principle, I agree...
Elves-Dark Elves also doesn't quite fit - after all, Dark Elves are descendants of ordinary Elves (they simply changed their beliefs) and have never tried to destroy their own kin...
Mages-Undead - it's essentially the same as with the Elves... And the fact that the Necropolis faction wants to destroy the Mages is natural - after all, they were treated so cruelly by their own people (the Mages) (read the history of the races)...
Dwarves-Barbarians... I don't see any analogy here - Dwarves are not mages, but blacksmiths - masters of the forge, of weapons... Barbarians are brutal and anti-magic... (the only possible conflicting difference is that Dwarves, in theory, strive more for technological advancement)

In general, the yin and yang principle doesn't work - otherwise, there would be 3 races on each side in the original game, but instead, there are 4 on one side and 2 on the other... I would declare Elves-Dwarves, Mages-Barbarians, Humans-Demons as opposites... but then the Necropolis faction and Dark Elves would be left without a pair...