The problem with Sigurd (and other Japan-only charcters) is that Sakurai said he's trying to avoid adding characters that are Japan only, and while it's likely he will add one or two anyway, I doubt it would be a japanese FE charcter seeing as there are plenty of international FE characters that can be used intsead. Sigurd also doesn't offer much beyond popularity (and Sakurai has said that simply popular charcters don't interest him as much). Seeing as Marth is more than likely to return, there really is only room for one more Blue haired FE swordsman if even that, and Ike has not only the worldwide recognition, but Sakurai has shown much more interest in him more than Sigurd. And while Sigurd is te most likely of the japanese FE lords, I really wouldn't say his chances are that high.
Lyn's only real drawback is her lack of support in Japan, which while is quite serious, is countered by her immense popularity internationally. Out of all the swordsman Lords, she'd probably be the easiest to make unique (though Ike also has a very interesting style). And seeing as FE7 was the first FE game to become international, then I think that there is a chance that one of the lords from that game will make it.
For other FE characters, I think other possible characters are Hector, Ephraim, Ike, Mikaya and Sothe. Hector and Ephraim are obvious, as they are the only Lords who use an Axe or Spear as their signiture weapon, so offer uniqueness that no other lords can offer. Mikaya is a mage and therefore would be incredibly unique, and seeing as it's likely she is the lord of the next game, that further boosts her chances. Sothe would be an interesting charcter, though his chances depend on how big his role is in the next game.
Ike is an interesting character, as he so far remains the only lord who is known internationally to have incredible popularity in Japan. Like I said, Sakurai has shown a lot of interest in him, on the same level as Ridley. Diddy and Geno. While appearance wise he looks like a mix between Roy and Marth, his sword style seems different enough to work (as his style seems a lot more Rude and rough than that of Marths).