What if

The way that I’d imagine a scenario in which Dawn reappeared, in the XY series specifically, is that Dawn would be going on a hiatus or sabbatical of some sort from contests, much like Zoey had regularly done, which she proved was an essential and effective approach in her pursuit of the Grand Festival. Kind of like a “mental-health break”, but essentially a vacation with Johanna to get away and recoup. Maybe Johanna owns a vacation home in Kalos and Dawn is trying to find new ideas to showcase her Pokémon by exploring the fashion scene there.

Fast forward to where Ash and Dawn reunite (however way that would happen): Ash and Dawn go nuts, Piplup and Pikachu go nuts, then some obligatory Pikachu and Buneary moment. I’d expect (or want) it to be kind of reminiscent of their reunion in Best Wishes, but more heartfelt. (<--That would REALLY make the Amourshippers’ blood boil, am I right? Lol) Clemont and Bonnie are thrilled to meet one of Ash’s old friends (Bonnie does her usual running gag of playing Cupid between Clemont and Dawn much to Clemont’s dismay, and she’s quickly reprimanded by him).
Ash and Dawn share an epic, thunder-clapping ~yet charming~ high five, and who’s there to witness it with a front row seat? You guessed it.

Steam coming from Serena’s head, furrowed eyebrows, gritting her teeth, clenched fists… One could only guess what’s running through her mind at this point. (I’d imagine it to be something along the lines of “ASDFGHJHGFDXFTGYHJKJUYTRFKJHGFD”).

Anyway, bottom line, I’d imagine that Serena would be jealous… but only at first. I mean at first impression, I’d suspect Serena to just be so utterly jealous of Dawn. Dawn, to me, and at this point, embodies everything that Serena is not (Yes, I know that we’re only 30-something episodes in, but for the sake of the argument I’m going to disregard that). They’re just in completely different leagues, and Dawn stands at the apex of the food chain.

The way I see it, there’s two ways she’d probably handle the situation at first.
1.) Become green with envy and make a constant effort to come between Ash and Dawn, and treat it as a competition, or
2.) Her heart would collapse and she’d just wither away…

The thing is, jealousy is just such an unhealthy trait when it comes to relationships. It is solely counter-productive and nothing good would come out it. Instead, it would poison her soul, so-to-speak. Serena’s jealousy just translates to how much she doesn’t trust Ash.

On the other hand, Dawn wouldn’t even get jealous by the interaction Ash and Serena would have with each other. She would be HAPPY even, and attribute their spending time together as a sign of a beautiful friendship. ‘Cause that’s who Dawn is! She is so optimistic and intrinsically takes the high road. It’s just the nature of her beautiful character. Also, Dawn is just as dense and aloof as Ash is (citing one of their many mirroring qualities), so any hints of romance between Ash and Serena would go right over her head. But what’s most important is that Ash and Dawn have that established trust with one another (as even evidenced in High Touch), something that Serena doesn’t yet exhibit, as I alluded to earlier.

Fast forward a bit, a few episodes go by, and I think it would just be happenstance that Serena and Dawn would develop a bond with each other and realize they take interest in many of the same things (fashion, good hygiene, baking… etc.). I mean, just by the nature of the show and it’s perpetual theme of friendship and such, I think that’s how things would play out between them. And you know what? I think that would show a great deal of maturity and character development on Serena’s part.

I think she would eventually outgrow her jealousy of Dawn, and I’d dare to say that she’d even go one step further and actually outgrow her crush and possessiveness of Ash, as well. THAT would be an impressive showing of events.

Time goes by, and (if it were a story I were telling) Richie from Kanto and first seasons shows up doing whatever he’s doing in Kalos and Serena starts to take a liking in him because he’s basically a mirror image of Ash.

Well, that’s how I’d imagine it…

Serena's definitely getting jealous. But she wouldn't bump heads with Dawn because Dawn will be super friendly to her, similar to Ash and Kenny.

I agree, Journey, Serena would probably be steaming in jealousy because of Dawn, even if it's on the inside! I think the real question is if Dawn will make another cameo in the XY series specifically.

A simple yes.

I like to think there would be a lot of jealousy from Serena's part, maybe concealed from the couple & definitely obvious either way. It would be interesting if Serena openly hated Dawn but acts like nothing is going on to Ash, & there's some catfight that goes on every time his back is turned.