Y'know, I've heard about the argument that Lucas could become another Roy, but in all honesty, I don't think it'll happen. Roy was arguably replaced with Ike and Lucina (I don't think that's the case, but that's as may be) and is widely agreed to be among the most unremarkable heroes in his own series; while he is a Smash veteran and a great character in his own right, I'm not sure Sakurai would be willing to bring him back, especially considering there are plenty of arguably more important and interesting Fire Emblem characters that could be chosen, and plus there's the matter of Fire Emblem having four characters in the first instant...
Regardless, Lucas is distinct in his own right: he can't easily be replaced or overshadowed, considering Mother is a much, much smaller series than Fire Emblem and there's only so much room to maneuver with regards to character choices. He's also got the added benefit of being a popular veteran with a distinct moveset that plays almost nothing like Ness - to regard him as "just another Ness clone" is to utterly disregard the sheer breadth and depth of his abilities and talents. I think he's a reasonably comfortable choice for DLC, all things considered, and I do hope we get to see him back in the fray!