Old discussion but it took me like half an hour to type it so here:
I love how just because of Awakening people who never even knew about Fire Emblem, now are basically shitting on it by asking that it's founding lord be left out over two reps from the same goddamn game.
The lords of Fire Emblem games are the following, in order:
Marth
Alm, Celica
Marth again
Sigurd, Seliph
Leaf
Roy
Eliwood, Hector, Lyn
Ephraim, Eirika
Ike
Ike, Micaiah
Chrom, Lucina
Now obviously I didn't mention the remakes, because they're remakes. I mentioned all the ones which I remembered from the games.
Marth is getting in, he's the titular lord of the entire goddamn series. People asking for Lucina over him are fucking insane. Now I'll admit that I haven't played Awakening yet, so I don't know the specific properties of each character, but given what I've read about Lucina. She can either either be an alt costume or if they want to play up Awakening's pairing thing then she can be some sort of tag team with Chrom, which would be pretty fun in my opinion.
As for the other lords here's a better look at them:
Marth, the first lord and the only lord to have appeared on more than 2 games IIRC. He's the fucking first lord, of course he's in goddamn it.
Alm and Celica come from Fire Emblem 2 or Gaiden. Almost nobody cares about them and they only qualify as lords due to technically being the protagonists of their game, so they have no chance.
Sigurd and Seliph are from Fire Emblem 4, which many I've heard many consider the best of the series. It is very important mechanics wise since it was this game that created the weapons triangle, and also was the first to have the relationship/children mechanic that many of you seem to love in Awakening. Last time there was a smash poll for characters, Seliph was one of the most requested Fire Emblem characters IIRC. Him and Sigurd are apparently some of the most popular lords in Japan. I don't know how if this feeling has continued, but I feel like this lends to him having a better chance than some other lords, not that it'd make him a for sure choice. I think that given the fact that his game hasn't released worldwide officially his chances are slim but still better than some of the other characters. Something unique about at least Sigurd is the fact that I think he was the first lord to ride a horse into battle and he could use lances as well. This could make for interesting moveset if it were to incorporate both things. Seliph can also use them after he promotes.
Leaf is from Fire Emblem 5, known as one of the hardest games in the series. This game was the one that created current chapter formats, instead of huge sprawling maps like the ones in FE4, we got smaller ones that focus on the troops movements in a village and whatnot like the ones we have in more recent games in the series. Leaf also shows up in FE4, and while he did get a redesign and appeared as DLC in Awakening, something that Seliph also got, he isn't all that requested and as far as I can remember has nothing too unique that could contribute to making him a playable character. His game also did not come out in the west, best thing for him would be an alt costume of another Fire Emblem lord.
Roy is from Fire Emblem 6, the first of the series to go on a portable console also never released in the west. Alright so a lot of people are asking for Roy back due to his inclusion in Melee, but the fact is that he was only in the game due to advertisement reasons. As a unique character he brings little in terms of innovation. His attacks in his game are kind of explosion and fire based due to the properties of his sword, but in Melee they just made him and Marth very similar with little having truly unique properties that represented them well. I honestly can't see Roy coming back for a good reason. He's another swordsman and his game doesn't hold all that much importance looking back on it. He also did get a redesign and was released in Awakening as DLC, while I think he's more suited to being an alt. costume some people seem to want him out of pure nostalgia for him being in Melee.
Eliwood, Hector and Lyn all from Fire Emblem 7 the first FE game to actually be released in the west. The prequel to FE6 starring Roy's father, the important thing here ironically enough has nothing to do with him. Eliwood himself is just another unit like Sigurd and Seliph, and an unremarkable and unpopular one at that. If a Lord Knight were to be included it'd probably be Sigurd or Seliph. The focus of this game lies in the other two. Lyn appeared in Brawl as an assist trophy. Currently the only default swordmaster lord in the series she'd not only be an interesting addition due to moveset potential but she'd also help bolster the female ratio in the roster. Easily my choice of this game but another potential is a fan favorite, Hector. Hector is the third and final lord of this game, he's IIRC the only lord in the series to primarily wield axes and not a sword. He would make for an incredibly interesting playable character in the next Smash, probably playing like an axe wielding Ike. Outside of moveset potential the important thing about this game lies in that it was the first of the series localized in the west and so it's a source of nostalgia for many western fans who started with this one, myself included. Lyn was another who appeared redesigned in Awakening as DLC.
Ephraim and Eirika both from Fire Emblem 8 which was also localized. Now this is another of the few fire emblem games I have not played, but aside from my personal experience with it many people seem to regard this as the black sheep of the series. I believe it was the first game in which there exists multiple reclasses and branching classes. Eirika and Ephraim are two lords in that Eirika is basically just another Lyn but with blue-er hair. Ephraim on the other hand is the interesting one here as he's the only Lord that actually has the Lance as his primary weapon and not as a sencondary, mounted or otherwise. I feel as though, he certainly has moveset potential but due to his game not having all that much importance in the series as a whole, the chances of either one are small or at least smaller than other characters that have similarities. Ephraim and Eirika were also released as DLC in Awakening. I can see both being alt. costumes for Lyn and Sigurd or Seliph respectivelly.
Ike from Fire Emblem 9 and 10, both released in the west. Ike was in Brawl as a playable character and many people want him back. Honestly Ike in Brawl had almost nothing to do with himself from his own games. Ike was never a slow character and that was changed in Brawl purely out of not wanting to make another fast swordsman. In fact even in 10 where Ike has buffed up CONSIDERABLY he's still ridiculously fast, it's just that now instead of only hitting like a truck he actually looks like one to match his strength. Ike was the current lord and so that was the reason for his inclusion in Brawl, seeing as that is not the case now I feel like the only reason he'd be back would be to keep the character and his properties. I really like Ike but the fact that he was so different from his character in his proper games always bothered me. I would enjoy seeing him back if only because he'd be another FE rep but honestly I'd prefer reps who have more significance to the franchise. He was also released as a redesigned DLC character in Awakening.
Micaiah from fire emblem 10 which was released in the west. An interesting case to be sure, she's the only default lord that is actually a magician. Her moveset potential could surely be argued and I think she'd make for an interesting choice. She would also help increase the female count of the roster so there is that as well. Outside those two things though there isn't much more to her, she isn't exactly all that popular as far as I can remember, and since her game has passed and it honestly doesn't have all that much relevance in terms of mechanics and etc to the franchise I can't see her getting picked as a rep. She was also released with a redesign as DLC in Awakening. From these games I would really prefer Ike over her.
Chrom and Lucina from the most recent game, Fire Emblem Awakening which obviously was localized and is responsible for the sudden increase in fans of the games. Like previously stated, I haven't had the chance to play this game yet something which I really want to change soon. Chrom is another swordsman but being the current lord and the "face" of the current game I am confident that he could get in over Lucina in case they had to chose only one of them. Lucina, from the little I've read, seems to be another swordsman who in a Smash game would probably work exactly like Marth due to her "history". I can't understand all of these people asking her to replace Marth, it's just the most ridiculous thing I've ever heard of and completely impossible unless Sakurai is goddamn insane. The best thing they could do is work in a pair to kind of show the new game's pair up mechanic, that would be really interesting and honestly kind of the direction I hope they go into.
There, those are my thoughts on the FE franchise as a whole. Take it as you will, my choices would probably come down to Marth, Lyn, Seliph, Chrom/Lucina with the last being a pair.