I had to go with Froakie, of course. Greninja is one of the most badass starters since Charizard. From design to his movepool, I see this guy on almost every team in competitive play alongside Xerneas.
So far, my team is Venusaur, Greninja, Blaziken, Sylveon, Gengar, and Togekiss (I had to pull some SERIOUS STRINGS to get Togepi this early--and Togekiss is a monster Fairy/Flying type now). I might swap Gengar out for Gardevoir sometime, since my team was SUPPOSED to be full of Fairy pokemon to commemorate its inclusion in our metagame.
I cannot state enough how popular Xerneas is online. I've yet to see anyone running Yveltal yet, and if you fail to kill it before it uses Geomancy it's GG almost 90% of the time. Haven't gotten far enough in game to nab one of my own yet.
I'd say go for it if you choose Serena (I have--her name is Verylin), but be aware since you "are" a girl you will have to sacrifice a bit o' masculinity on the dress-up scene (mascara/eyeliner, lipstick, purses, etc). Depending on gamers of either sex, this might be a turn off...I don't mind it too much, though.