FE1-3 era characters that are likely:
Marth - No explanation needed.
Sheeda - She might not be a Lord, but when it comes to important characters, she does rank for that game series. She's the main female lead, I can see her making a spot if Marth does.
FE4-5
Sigurd or Celice - I'd rather they don't, they'd set a bad set-up for the characters to add them. They aren't new and they aren't classic, they are simply popular. So far all the Brawl characters are classic (Sonic, Mario, Snake, Pikachu, Kirby gang) or recent (Ike, Zelda cast in their new form, Peach, Lucas). But for their storyline they rank.
FE6-7
... Roy... he's it. Ike kinda undercut Hector, and Roy undercuts his dad.
FE9-10
Micaiah - She's Ike's costar, despite her roy-like popularity. By and by, I DO OWN RD, I DO PLAY IT, AND STILL LIKE MICAIAH.
Elincia, Mist, and Sothe - Similar vein to Sheeda, Elincia was the true "Lord" of 9 in the same sense that most of the Lordlings are exiled/escaped children of overthrown kingdoms. Mist is Ike's little sister and has unique standing in both 9 and 10. Sothe is to Micaiah as Sheeda is to Marth, not likely without Micaiah, but has a shot.
Soren and Ranaulf - Both of these characters share endings with Ike, technically plausable, but not by much. They are sorta Sheeda to Ike. I don't give either much chance.
Mia - LET ME HAVE MY DREAMS.
I didn't touch FE8 as nobody touches FE8 anymore. And while a few of my choices are unlikely, they are not impossible.