I think that the presence of Ike means that Marth and Roy are both out, and that the Dark Knight doesn't have much of a shot either. Why? Because Fire Emblem is a game that's all about matching different weapons against each other. The whole combat system is based around swords vs. axes vs. lances (not to mention the magic!). To have three swordsmen--not only that, but three characters wielding what is essentially the exact same sword--would be a complete waste of potential.
Unfortunately, from what I've heard of Fire Emblem (I've only played the first GBA one so far, though I'll be getting Path of Radiance really soon), the lords tend to be very sword-heavy, which makes it difficult. Hector, with all his axe-wielding awesomeness, seems like he would be a good choice, and even though Lyn has a sword as well, the facts that it's so small (relatively speaking) and that she starts with it sheathed would make it easy to give her an interesting, unique moveset that mixes hand-to-hand attacks with just a few powerful sword strikes (plus she has a bow).