Well, if its a pokemon for Serena, what will be it be. Get One can apply to all pokemon Ash owns (Save for Squirtle, Bulbasaur, Quilava and Gible for obvious reasons). I don't expect Serena will get a Pikachu, since i doubt they will let any other than Ash showcase Pikachu on the main cast. Taking the calendar into account, with Snorunt on it in January, it suspect it will be a Snorunt which will evolve into Froslass, which will be Glalies love interest and thus Get One. If not, surprise me. I would love to see Ash and Serena dance (But also Jessie and James finally declaring love to eachother).
I would love it if Serena caught a Snowrunt here or maybe this is where Eevee evolves.