be a clone of Captain Falcon.
Young Link, while still a version of Link, also has potential. My guess is that they'll either make him into Wind Waker Link, with similar A attacks but WW-inspired specials (Deku Leaf, Grappling Hook, Skull Hammer, etc). Or, they'll scrap YLink altogether, and add in Midna + Wolf Link, which would play radically different.
Same goes with Falco. Though a good character on his own, he needs a different focus than Fox - his similarity to Fox is the sole reason he's a higher-tier character. I can see him being made into an aerial specialist (he is both a bird and an Arwing pilot), maybe with different specials or different properties to them. Or, they could replace him with Wolf, who would also make sense as a Luigified Fox.
Even Pichu could be made into something of worth, as the Pikachu evolutionary line can learn tons of non-Electric moves that would translate well into Brawl, and would make the character that much better. Either that, or they make him an absolute killing machine with power and speed on par with, say, Capt. Falcon, but the same weaknesses as Melee. Of course, there's a good chance Pichu won't be returning.
Then, we get to Dr. Mario. Since a Luigified Mario is, well, Luigi, there's not really much you can do with him. However, since Mario is balanced, the Doc could be made slightly stronger but slower, but still relatively balanced; but his moveset, short of including needles and drills, can't change much.
And finally, Roy. Roy wields a sword, and no matter what they do to his swordplay, he'll end up similar to Marth, Ike, Meta Knight or Link; they could take a few moves from each character and Frankenstein Roy's new moveset together, but I would much rather see a more original FE character, like Sigurd (with both a sword and lance) or Micaiah (with light magic and staves) than a mishmash of swordplay.