Harems are harems. Might take place in a school for witches and wizards, everyone might be a mecha pilot, could take place in an alternate universe full of cat people, or they could all be monster girls. The core principle remains unchanged. 3+ girls (and maybe a trap) fixate and fight over a borderline a sexual generic male character who might super special to the plot for some moronic reason but has the intelligence of a brink with the personality to match.
Oh and Rosario + Vampire already kind of did the monster girl harem thing before..