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

You are right about the Efreet. The Nagas are already being overrun by Rakshasas, and if you create a new creature for each unit with four arms, it will look ridiculous. Four arms are enough for the heroes. We definitely need to bring back the monsters and the werewolves. What about the one-eyed creatures? Cyclops, perhaps? They will be there. And if you don't make a swamp, I'll start haunting your dreams!

In general, the principle of Yin and Yang doesn't work - otherwise, there would be 3 races on each side in the original, but instead, there are 4 on one side and 2 on the other... I would consider Elves and Dwarves, Mages and Barbarians, Humans and Demons to be opposites... but that leaves the Necropolis and the Tower without a pair...