Romance is not just about get the couple in bed and slam, bam, thank you ma'am. Romance is about relationships. When writing a sequel, I visualize the couple in a different phase in their relationship. Say they got married and now have kids, their relationship is not the same as when they first met. They have different obstacles they face, whether it's less time together or someone focusing on their career instead of the relationship. Personally, I view it as real life situations, no relationship is going to be focused just on beginning love. The couple needs to evolve and change, if not the relationship is stagnant.
I think whether a sequel is romance or suspense you still need to change up the dynamics and situations the characters are involved in.