If Contest return, I prefer that Cilan or a new boy will take that role. Because I don't like a third girl doing Contests.
But if they consider doing Contests again, I prefer Dawn going into Ash gain, explain why she want to return to Ash (although could be a ridiculous reason, fly to Poke-France and voila...).
Why? Because I like Dawn. But because:
-In the Best Wishes merchandise (I was right, BW merchandise) Piplup has gotten in the fall-winter, the 5th pokemon in merchandise appearances having a way lot of more appareances than Axew. THe only pokemon that has gotten so much are Kyurem FOrmes, Oshawott, Eevee and of course, Pikachu. (Axew is around the 20th on merch behind pokes like Snivy, Emolga, Dragonite and now behind Charizard..., if you think that Axew is too marketable, you're wrong). The first person that will be happy if Dawn returned permanently will be Takara Tomy, Bandai and Banpresto headquarters in Japan.
-TV Tokyo, CoroCoro, magazines, intended that Dawn was returning to the main cast. If you see the promotion of Season 2 you know why. Remember that "poster" and that character pages each time we looked in pokemon.co.jp last summer.
If the Pokemon Contest really return, why not bring back a character that was popular at the time and that the audience knows because she had make an appareance 1 year ago? It's true that she promotes DP and that her Piplup is a starter for DP, but I prefer bring back a popular character with a very proven marketable pokemon than a unknown character with an unknown pokemon doing the same role.
I don't think that this will be happen but who knows. THe anime is getting rare recently (trust me) and the companies need to get the 120% of merch now.