Mewtwo is going to be a lot stronger in this game competitively. Not necessarily due to buffs, but due to the changes in game mechanics.
New ledge mechanics would make Mewtwo a lot harder to edgeguard in this game, especially since he has the fastest teleport out of the entire cast. Additionally, rage also helps Mewtwo out. While he is on the lighter side of characters in Melee, he still has a small amount of bulk to him. Which would potentially allow him to live to percentages where he could implement rage and convert it to kill throws (ala Ness). And, like Ness, Mewtwo has some pretty strong throws.
Lucas has a lot of Ness's strengths, but is better and worse in some areas. He has a faster Fsmash, but he lacks a Dsmash that hits behind him. His Dair is better, though Fair can be argued to be a bit worse despite still being good. His Bair is a spike and his Usmash has scary KO potential, but his throw strength is weaker than Ness's iirc. He does have a tether recovery though, and his specials are overall slightly better than Ness's. He also has faster running speed which would help him out.
So overall, both of these characters have the chance to be quite good in Smash 4. Mewtwo is probably not going to be lower than Mid tier, and Lucas is probably going to have relatively the same amount of strength Ness possesses. And Ness is pretty great in this game.