I'm doubtful that Pichu will make it back. Marth too. If either of the Melee FE characters will make it back, it'll be Roy, because Marth looks THERE IS NO FUCKING E IN RIDICULOUSly similar to Ike.
But then again, Lucas looks THERE IS NO FUCKING E IN RIDICULOUSly similar to Ness, so if Ness is in, why not Marth.
Roy was there to promote FE. That game has already hit its peak sales and has dropped off since. Marth on the other hand was THE first FE Lord. He's an icon in the FE series. He'd have a much better chance at coming back than Roy.
Mario looks like Luigi. Luigi will end up coming back. Kirby looks like Jigglypuff. They're both still in the game. Pichu looked like Pikachu, but he was in Melee. Fox looked like Falco. He was in. Dr. Mario looked like Mario. He was in Melee. Marth looked like Roy. They were in Melee.
The look-a-like argument isn't that good or convincing. Besides, if the original 64 cast comes back, that means Ness will look exactly like Lucas, so a character like Marth shouldn't have trouble coming back even if he does resemble Ike.
On a side note, all the FE male characters look the same. The main ones anyways.