Them being Dr. Mario, Pichu, Mewtwo, Young Link, and Roy. Let's talk about them and their chances.
First off, let's look back to Brawl. In particular, the partial data. This does not bode well for Young Link and Pichu, the former who has an obvious replacement, and the latter who, honestly, seems rather pointless. Dr. Mario, Roy, and ESPECIALLY Mewtwo have partial data within Brawl. Anyway, I'll put out my opinions, in order of best chance to worst.
Mewtwo probably has the highest chance. The reason being his partial data is the most complete, and if I recall him being extremely popular. If the game isn't rushed, then I imagine that there will definitely be an appearance from number 150.
Oh Roy, our boy. Again, he DOES have partial data. The fact that he was in Melee at all gives him a helping hand over other Lords who aren't named Marth or Ike. In addition, his new redesign gives him a fresh look, and also adds an obvious final smash for him. Pulling out the Sword of Seals. On a side note, since Ike has his own way of fighting, I wouldn't count him out.
This is where it starts to get hazy. True, he does have partial data, but there is the fact that he is a clone of the most obvious variety. After all, he is literally Mario in a new set of clothes, you have to think that he'll have trouble bouncing back. Maybe he'll come around on his own, but I think the best chances for him would be appearing Wario-style, as a second set for Mario.
Meanwhile here is where it starts getting hopeless. In a way, he does have a high chance of coming back, but not inherently as Young Link. In Brawl, of course, he was replaced with Toon Link. The main question about him is 'do they want to keep Toon Link'? If we consider the "Toon Zelda & Sheik" data, then that would be a probable yes. However, the most recent game with a Young Link is Ocarina of Time 3D, which has come out after any toon Zelda games. His chances are slim, but stranger things have happened.
And here it's completely hopeless. The main gimmick with Pichu was a faster, lighter Pikachu that hurts himself, wasn't it? In all, there wasn't a reason for Pichu to exist, and there certainly isn't one now. If something comes that'll serve for him, it'll likely be a different electric rat, or a different Pokémon entirely. Really, I have my money on Raichu.
Lose one, let go to get one
Left one, lose some to win some (You lost one)
Story of a champion, sorry I'm a champion
You lost one
I'm an ass man and I love all kinds of asses
Lose one, let go to get one
You might win some, but you really lost one
You just lost one, it's so silly, how come
Hey, you ever coming back?
You kidding? I am Queens Boulevard.
*Burps real loud*
I don't see any reason why Roy would actually make a come back in Sakurai's eyes.
Show me your moves!
The only one really justifiable is mewtwo because he actually mattered to his series
I am probably one of the few people here who really liked Pichu, and mained him.
RAW on July 16th, 2012!!! WALUIGI TIME!