With how Serena left the series, I always thought that a return episode would be tricky to handle. I don't see how you could have a reunion of her and Ash without addressing their potential relationship in some way, between Serena's last words to him, and of course the kiss. She essentially confessed to him through her actions there, while at the same time saying that she wanted to go off and improve herself before pursuing any kind of relationship between them. If you just bring her back for a few episodes without addressing that at all, you completely invalidate the subtext of that scene.
Now, if they actually paid attention to all of that, and wrote the episodes correctly, I think it'd be really interesting to explore. But somehow, I feel like that isn't going to happen, at least not any time soon. If they give Serena anything within the next few years, I think it'll be more of a special focusing on her alone, and perhaps some other returning character, like the Brock/Cilan episode.