RTF is a global modification that replaces all towns in the game.

Well, if you don't try to be overly ambitious and don't make each map based on the principle of "the bigger, the better," with tons of additional quests and cutscenes like a full-fledged movie, then I wouldn't say it's something extraordinary.

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Aren't liches and vampires a good fit?

You can take liches from Chaos Theory

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and, if you want, you can even pull more liches from Disciples and create an alternate version (Astral can help with this, as he knows Disciples very well).

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Regarding spiders: can't they be made on Cerberuses? A spider could spit poison or web, and the death animation is suitable; you just need to attach the legs, and otherwise...

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If you make vampires occupy 4 squares, they need to be large, really huge, like Landsknechts; then they will look relatively normal, or medium-sized, but with some kind of cloak, etc., so that half of the squares aren't empty.

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Regarding the plot: you can create a couple of non-intersecting campaigns that intertwine at the end, like in the original Heroes 5. First, there's Isabel, then Agiroil appears at the end, then demons and humans appear without any connection, and then Markel appears, and only with the elves does there start to be a relative intertwining. Before that, the plot lines only slightly touch each other