Characters I'll main:
Diddy is obligation. Plus, I wanna proof people he can still be formidable without Brawl gimmicks. DK's buffs are awesome from what I've seen, and him being either a main or a secondary in all previous games should really put him in here. He always gets bigger, faster, and stronger to!
Secondaries:
The reported changes in Link and Samus really interesst me. Plus, am very happy that Ike is back, and Charizard is solo this time. Hopefully they'll buff both reasonably enough, as they felt kinda clumpsy in Brawl. However, with the changes Bowser has gone under it has to be expected. Ike and Charizard aren't gonna be slower than him.
Potential secondaries (depending on playstyle) :
Pikachu has always been a character I liked to use in all games. Outside of Melee really, cause as a kid I couldn't get over his nerfs and moves on to Mewtwo and DK. If this didn't happen, Pikachu would've been a standard secondary, behind only Diddy and DK. However, he's the one character here that I feel has most potential making it as a secondary. Thunder seems interessting. QAC is back. Promising character for sure!
Toon Link am just very happy is back.
Sheik, while I did wanted her to be cut, seems like she has significantly buffed. Out of spite for the character, I picked her up in Brawl and then Melee. Her playstyle is cool, and she "MIGHT" be the closest thing to Impa I'll get. If Impa is out, I guess I'll try her.
Yoshi getting buffs and actually being somewhat viable will definitely make me use him casually.
Marth has always been a character I wanted to be somewhat good with. In Brawl, this was easier than Melee I felt. However, I did gain some skill with both incarations that I have some confidence in. If he's still good, I will at least play him casually. Plus, I really enjoyed Fire Emblem 12! I really did.
Veterans I also feel like trying:
Ganondorf: If he has undergone changes, he'll probably be a secondary.
Ness: Loved him in Brawl. Will try out. Hopefully will have significant buffs, and wouldn't be opposed to new moves either.
Zelda: Complained all the time that she should be separate. Wanna try out some Phantom shenanigans. But she'll probably suck.