I'd say "the idea" must never force a genre. Or favor one (too much). So anything that can't be precisely pinned to any officially recognized genres (better to just have a list setup), or doesn't exclude any particular genere, would be fine. By going with a definition of what you want you can avoid corner cases, or "interpretation of the rules".
For example, going by your words Illu, if you say "it must include cherry trees and puberty" you force a romance genre. But if you say "it must include vampires" that could be anything. Both of them can be considered "themes" or "ideas" or what chu'wanna call em.
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