Maybe when Serena and Elijah see each other again they both admit their feelings for one another and have their first kiss which doesn't apply anything about their relationship status but shows their true feelings? I'm not sure. But you could show how much Serena cares for Elijah by saying how heartbroken she is without him once he gets daggered. These are just suggestions, you can take the story wherever you want and I know I'd love it! Xx