I don't get the interest with Anna or even Robin, but I'm can't be objective since Awakening is a weak entry is the serie. Robin could be a good counterpart to Zelda (magic + sword)but he's nothing but blank to me.
Rekka no Ken aka Fire Emblem is the first FE game released worldwide so it's maybe the most renowned of them. The game is flawless, 3 great Lord. But since the Blue Lord /w Sword gate (Marth/Ike/Chrom), I just hope Sakurai will think about it ... one day.
Oh, and I hope the game will have a lot of diversity in term of 1 playee game mode because MK8 is already boring... Even online...
Rekka no Ken/Blazing Sword is my favourite FE game (and one of my favourite games of all time), and Lyn is my favourite Lord, but I think at this point it's not reasonable to assume they've got any sort of good chance of being viable candidates as Smash characters anymore. It's been too long, and no matter your or my personal feelings on how good the game was, I really don't think the mainstream would agree with your assessment that it's the 'most renowned'. It's good to be realistic about things sometimes, and Lyn's already been an assist for two games straight. I think that's the closest she'll ever get to a mainstream Smash game's stage.
And I feel like I've been repeating myself a lot on this subject (I should just copy and paste something at this point, lol), Anna doesn't really win points because of Awakening itself, she wins points because of pretty much every single other game in the series. She's one of the constants of the franchise, and effectively the unofficial mascot of the series for that reason. She's a pretty effective catch-all representative of FE as a
whole (where Roy, Ike, and arguably Marth were more singular representatives of their respective games), and therefore can borrow from FE as a whole in terms of diversifying her moveset. And as Azure mentioned, she also acts as a pretty handy representative of the aspects of FE that Smash doesn't do a great job representing.
And aside from her own qualities as a fighter (a 'trickster' fighting with a backhanded sword, stave, and tomes if you wanted to push it), her role as a merchant puts an interesting angle on what she could look like as a character, conceivably having access to every single weapon and item in the game. There's just... a lot of material for Anna. More than most FE characters.
In short, there's a lot of reasons why Anna makes sense as a Smash representative, and in any case, I certainly see her as much more likely than most non-Lord or GBA-era characters, and would much prefer to see her over Lucina as she would, gender-aside, just be yet another blue-haired sword-lord.
I'm not totally enamoured by Robin, but strictly from a moveset standpoint, s/he's a tome user character when all the other representatives of the FE have just been sword users, and tome magic would obviously do a lot to distinguish them from the existing roster and I'd be cool with them for that reason. All the more so if gender were toggleable.
The key here is really diversity within the context of Smash's existing roster, when it comes to what I actually want to see. I still think it's ridiculous that FE has no female representative when it's historically had some of Nintendo's best female characters, and the series insofar as Smash can do a lot better than a narrow criteria of 'blue guy with a sword'.