Although I believe Chrom would be a worthy addition to the roster, I don't believe he is the best addition from his game. First off, Fire Emblem: Awakening has risen the profile of the Fire Emblem series to new heights, returning its popularity in Japan and increasing its popularity in the west. It also has been the series' most critically acclaimed game that received a world wide release. And it will, if it hasn't already, be the first in the series to reach the one million sold mark. As such, I believe the series is positioned to have three characters in the next smash. Naturally, there will be an awakening rep, and by my argument, the other two characters would have to be Marth and Ike. Roy would not be likely to return since 1) his game never came out in anywhere other than Japan 2) adding a clone would not follow the four rules Sakurai put out. Marth and Ike are easier to put in than say Lyn, Hector, Eirika, etc since they already have movesets and playstyles already designed.
(Spoilers for Awakening)
With that in mind, that leaves the third Fire Emblem Rep to be between these four: Chrom, Lucina, Anna, and Robin (Avatar). Robin seems unlikely for a bunch of reasons, such as it's a customizable character (you could hood him/her, but that takes away from the character). There's also a question of the voice since that was also customizable. Due to this, Robin doesn't appear any promotional material to the game, so it's hard to get excited for the character. Anna would be an interesting choice, but her noteworthiness to the series comes from her being a shopkeeper as opposed to a swordfighter, she would be recognized for the wrong reason. Also, it is hard to argue putting her in front of the game's main protagonists.
Main protagonists? Yes, Chrom and Lucina are both the main protagonists in the game (as well as the avatar). Although the game gives you the option to not to use Lucina, there are two reasons why this is: 1) You receive her at a lower level and at the middle of the game, it would be unfair to be force to use her and then have her die. 2) All prospective mothers are able to be killed and they simply retreat from the battle aside from the avatar. Lucina is a main protagonist since 1) she's integral to the plot 2) she has conversations with the main bosses. 3) She has a unique conversation with Grima. To not look at Lucina as one of the main characters either means you haven't played the game, you didn't use her, or your favoritism towards Chrom does not allow you to see her as a main character for the sake of an argument. You can argue she's a deuteragonist or a tritagonist, but she's a protagonist nonetheless. She's a lord, and being or becoming a lord is a sign that you are one of the protagonists.
As such, Lucina fits the criteria of being a playable character, if less of a chance than Chrom. However, in the promotional material such as the soundtrack and the artbook, she appears on the cover. This is either because she is more popular in Japan than Chrom, or she simply would sell more copies. Either way, this indicates her importance to the game, as she also appears in the middle of the cover art for the game. So there is a possibility she is more popular than Chrom (in Japan). She is the protagonist of the children generation, which plays an equal part of the story, should you choose to use them.
It's this popularity that make her a better choice than Chrom. Choosing Chrom would give the series just another blue haired male sword fighter, which would be an improper representation of the series. Lucina would also add another female to a roster in need of more diversity.
Personally, I would prefer a pairing of Chrom and Lucina that shared a percentage gauge, but that you could switch between. The one in the background wouldn't attack but defend when it comes to shielding. The one could be singled out like the Ice Climbers, but when attacked it would not receive any damage unless it was in the main position. But I don't think this is likely so I'd rather root for the character I like better in the game.