For most Overrated and Underrated, I'm going to assume that the final roster is roughly 72 spaces in a 6 x 12, 1.5 x as tall and as wide as the demo roster with character portraits scaled 2/3 the size in both dimensions without changing the portrait's length or width relative to the other. I'm assuming there's only a few remaining spots for both echoes and newcomers. I think I'm going to do a most overrated and underrated for both echoes and normal characters if it's allowed.
Most Overrated Newcomers:
1. Decidueye.
With Squirtle, Ivysaur, and Pichu all returning with Smash 4's full Pokemon cast, I'm just not seeing this guy as likely anymore on top of all the other Pokemon we have in this game. We have 3 links and 2 pits also as archers, Ivysaur has taken the grass type niche that was once exclusive to it, and the fact that Mimikyu and Lycanroc have not been confirmed as pokeball Pokemon either further dampens his chances among the same generation. This character really needs a rerate, they had a roughly 58% chance (the second highest) the first time they were rated, and I highly suspect this number would drop drastically if they were to be re-rated to be more in line with lycanroc and mimikyu.
2. Captain Toad
With Toad having increased emphasis in the movesets of both Peach and the newcomer Daisy, Captain toad is in my opinion pretty unlikely. Toad shows up in both movesets in a full bodied form for both neutral B and grabs. Doc was further decloned and Daisy was added as an Echo as well, so the Mario series has definitely had some attention given to it for this game. I'm pretty doubtful Captain toad's gonna make it in this time although I think he would be very interesting as a playable fighter, even though I understand he has some defining features setting him apart from most toads.
3. Celica
I'm just not seeing it with how many fire emblem characters were in the smash game, and the likelihood a fire emblem character from the upcoming switch game is DLC or the possibility of Chrom being an echo fighter. Celica is an interesting fire emblem rep, but I just feel like she's not going to make it in this time.
Honorable Mentions:
Mimikyu: With pichu returning, I'm really uneasy about the idea of 3 playable characters who look like pikachu, and think it's unlikely, but I could be mistaken as there are 3 links in the game.
Paper Mario: Same as above except for Mario instead
Crash Bandicoot: A new arrival to nintendo systems, I don't think he's going to be in the base roster. Would have likely been an E3 announcement if he was in in my opinion. Spyro also is in a similar position but even less likely, and we already have a purple dragon in smash bros now.
Rex and Pyra: Kind of unlikely IMO for base game, designed a bit late and the voice actors don't seem aware, almost guarenteed DLC though.
Most Underrated Newcomers:
1. K Rool
I honestly think it's extremely likely we get a DK rep. Whether or not it is going to be Dixie Kong, K rool, or both is up to the imagination of the designers and speculators, but what we do know is that after Ridley, it's extremely likely DK gets the next representative. Mario, Pokemon, Zelda, Fire Emblem, Metroid, Star Fox, and Kid Icarus most likely having a satisfactory number of characters for this installment, I highly think that DK is the most likely franchise to get a new character at this point in time. Either is possible as a Unique fighter, but I feel like K Rool is more likely due to reasons listed further down.
2. Chorus Men
With the ice climbers returning, and the assumption that the rhythm heaven symbol in smash 4's datamining most likely belonging to a character, and the gematsu leak being highly implied by Sakurai statements to have been a legitimate leak of an early-development roster plan (implied by statements about Chrom and other interviews), they are easily the most likely representative of a new first party series in smash bros. Most likely similar to the Ice climbers, but being 3 instead of two, they seem likely as suspects for this game. I'm shocked they were never rated before (although there was a vote for the likeliness of a rhythm heaven rep, and that was decently likely.)
3. Simon Belmont
Vergeben is probably right, and there's a very off chance he's wrong at this point. Although he was wrong about the new castlevania game, there is a chance it could be announced in a future direct. Most likely third party newcomer as of right now in my opinion, as Bomberman as an assist trophy and Snake returning signify konami is working with Nintendo. His whip and history bring a lot with his uniqueness as a character.
Honorable mentions:
Elma: Unsure whether it's Elma or Rex and Pyra, depends on timing and planning. I do think Rex and Pyra are almost certainly going to be DLC characters if they aren't in the base roster regardless.
Bandana Waddle Dee: Most likely Kirby rep, and Kirby is the series most likely to acquire a new rep after Xenoblade and Donkey Kong in my opinion.
Rayman: Most likely representative of Ubisoft, who is on increasingly favorable ground with nintendo as shown by E3.
Geno: Popular request, Sakurai likes him, Square Enix is working with nintendo, and mii costume last game. Quite possible in my opinion.
Ashley: No longer an assist trophy, comes after Kirby/DK/Xenoblade though imo.
Most Overrated Echoes/Clones:
1. Galacta Knight/Dark Meta Knight
They've got alt costumes on Meta Knight already that are very similar, but not identical to them. I don't think we'll be seeing them.
2. Dry Bowser
I love Dry Bowser but I don't think he's as likely as people seem to think he is following the Echo Fighters announcement. Maybe as a costume? In the ideal world, I would love if he just inherited bowser's old moveset from brawl/melee with beefed up power, but I know it's unlikely.
3. Chrom
Just because he's not in the demo doesn't mean he's not still in Robin's final smash. Not outright unlikely but some people seem to think he's very likely. I understand he's a popular character but it's quite likely he's still in Robin's final smash until we have more information.
Honorable Mentions:
Dark Link/Spirit Link
Dr Luigi
Funky Kong
Most Underrated Echoes/Clones:
1. Dark Samus:
Her samus costume and Assist trophy don't appear to be in the demo, and she definitely shares some of samus's characteristics as an assist trophy in the last game. Canonically dead though.
2. Dixie Kong
Controversial opinion for sure, but Diddy Kong's tail attacks were completely removed, so I think she'll possibly be an echo fighter of Diddy Kong with a special recovery. This is assuming she's not a playable unique newcomer and K Rool is instead, fully acknowledge I could be wrong here.
3. Impa
The Most likely Zelda echo for sure for shiek, would be interesting to finally see her implemented into the game.
Honorable Mentions:
Blood Falcon
Majora's Mask young link?