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The new faction should include creatures that have never appeared in HoMM before, which would make it more interesting. However, at the same time, these creatures are present in classical mythology, for example, sphinxes (as is known, there were several in mythology), patriarchs who resurrect fallen warriors, archons (the first of the earthly elemental lords), mermaids and tritons, sea serpents, and so on.
By the way, speaking of holidays, we can delve deeper into some of them and find something interesting there to create a new, "festive" faction. Let's take Halloween, for example: pumpkins with carved faces, witches, ghosts, werewolves, and other evil creatures.