Building the world the protagonist enters
Portal romance depends on the other world being genuinely different from the protagonist's world and genuinely real as a place. A world that is different only in aesthetic terms — different architecture, different clothing, different magic systems — is not fully built. The world needs internal logic: how it is governed, how its economy works, what its social hierarchies are, what it values and what it fears. This logic needs to be present in the story even when it is not explicitly explained, because the reader needs to feel that the world exists rather than that it was constructed for the protagonist's romance. The love interest who belongs to this world should be shaped by it in ways that are visible in who they are — their assumptions, their reflexes, their particular relationship to the world's specific rules.