I am only a few chapters into Awakening (I really wish I'd finished it by now. Stupid college.), but from what I've seen, I would agree that Robin, and not Chrom, is the central character.
That said... who do you guys honestly believe Intelligent Systems are most likely to choose to represent and promote their game?
Robin would make for an interesting fighter, possibly more so than any other character in the game, since he/she can wield weapons and cast spells, but being a player avatar whose face, hair, voice, name, and even gender can be customized might hurt his/her shot at being the mascot-type rep of the game. Then again, Villager proves that may not matter.
Still, Chrom and Lucina are the two most prominent characters on the box art, and are therefore the characters people are most likely to associate with that game. I have a gut feeling they have better chances than Robin, as far as promoting the game goes.
Marth is the series mascot, the original lord, starred in his own DS game a few years back, and was an important part of the story in Awakening (Chrom's ancestor, and the alter-ego of a main character), as well as a playable Einherjar.
Ike is a fan-favorite, is a main character in two FE games, was more distinct from Marth than Roy, and was also an Einherjar unit in Awakening, so I could see them asking/allowing him to return in Smash 4.
If they do indeed want a new fighter unique to Awakening, Chrom or Lucina seem more likely as mascots than Robin. I almost feel like of the three, Lucina is actually the most likely Awakening mascot, especially if the developers want a female rep. Maybe not the best choice, but the most likely.
But I'm thinking as a fan, not as a developer. For all I know, Awakening may be represented by only a stage, or there will be four or five FE fighters.