Assuming this game is gonna be a new game and not just a port for some reasons that others have shared and also one other reason: If it was just an updated port like Mario Kart 8 Deluxe, then wouldn't it make sense to keep Link's Twilight Princess/Skyward Sword design? But yet, we see that he's now rocking that Breath of the Wild look. This leads me to belive that Link is getting a substantial tweaking of some sort to reflect this, which suggests that this game is overall gonna be substantially reworked.
Another point of interest; HAL Laboratory, the company that produced 64, Melee, and Brawl, is credited at the end of the trailer despite not working on SSB4, suggesting they have returned for this next installment.
As others said, we'll probably be getting some big cuts should this be a new game. My guesses as to who's staying and who's leaving out of all the popularly contested characters may not be all that special, but dang it, I still want to share 'em:
-The clones have always been bullied and harassed, and they are the most common targets for cuts (ex: out of all 5 characters in Melee that didn't return to Brawl, only one of them (Mewtwo) wasn't a clone). In SSB4, there are three full moveset clones; Dr Mario, Lucina, and Dark Pit. Out of all three, the Doc is the most likely to return, being the full clone with the most differences from his source character and having also appeared in another Smash game, but I still wouldn't hold my breath. Unless something crazy happens, then Lucina and Dark Pit are most definitely outta here. Best case scenario is them appearing as Marth and Pit alternate costumes, respectively, but that's not all that likely either, especially in Dark Pit's case (as he was given his own slot partially to avoid conflicts with canon), and as much as I'd want to keep scoring disgusting Side-B kills with my boy Pittoo, I'll probably have to face reality.
-Wii Fit Trainer isn't likely to come back, for all the reasons VioletSmashFan explained and the fact that there are many current Nintendo characters that could easily fill in the gap left by her absence, so she'll probably be the first cut after the Lucina and Dark Pit. Aside from that, I'd also say Duck Hunt Dog would not return, seeing how he's done very little even after his Smash 4 appearance.
-Other veterans and SSB4 newcomers. I actually don't think any of the original 12 are going. Why? Well, my reasoning is pretty much the same as RelaxAlex's in his "What if Smash had only 30 characters" video (basically, just from appearing in all the Smash games thus far, they've become highly recognizable to Nintendo history as a whole. They've been there since the very start, through all the ups and downs of the series, they have been there to see it all). Now, who else could be going? Well, let's start with Pokemon, the franchise with the most cuts in the series. Well, I think Pikachu and Jigglypuff are definitely staying along with the rest of the Original 12, and Lucario's staying for sure. Mewtwo is highly popular and the fact he made it back as DLC at all after being cut really says a lot, so I think he's going to be fine. Greninja though..... well, he was added before he got popular, making him a rather lucky choice, and with the new games released in the gap between Smash installments, it's reasonable to suggest he's gonna be replaced in favor of one of the new pokemon.
-Onto the discussion of the Fire Emblem characters. Even Sakurai
has stated that 6 Fire Emblem characters is too much, so he WILL be kicking some out. But who? Well, Lucina I've already explained above is probably outta here like last year, but who else? Marth is obviously staying, and Ike seems to be a mainstay now, so that leaves Roy, Robin, and Corrin. Roy is a bit more difficult to determine, but the fact that he made it back after being cut is impressive as hell, and similarly to Mewtwo, Sakurai probably wouldn't be making a smart decision by cutting him a second time, due to all the outrage it would cause. I don't want to jinx it, but I belive Roy's staying put. That just leaves Robin and Corrin. Sakurai put a LOT of heart and passion into both of these characters, so it's hard to imagine either of them getting cut so soon, but if a lot of space is needed, it's still possible that one or even both of them will get axed.
-On to third parties. Others have said this, and I agree; no third party character is a lock. Circumstances can always change. But surprisingly, I belive that not that many third parties are going, as most have some fair points in their favor and some reasons to stay. I feel it necessary to bring up Snake being cut from Brawl; his cut was likely due to the fact that in recent years, Konami's pub
lic image.... hasn't been the best. This hasn't been confirmed, but it's very likely why he's gone. Sonic is the most likely out of all of 'em to return, due to his popularity and the fact he's appeared in multiple Smash games already. Mega Man and Pac Man made it in SSB4 even though their series weren't doing a whole lot at the time, and due to their overall popularity (plus, Mega Man 11 being confirmed for the Blue Bomber), it's hard to imagine either of them getting cut, but it could still happen. Ryu on the other hand has few points in his favor, being an unusual addition to the roster (plus, even amongst Street Fighter developers, he seemed to not be the popular choice;
(ex:Yoshinori Ono would have preferred M. Bison over Ryu), but the fact that he actually turned out to be a very well received character in Smash and his unique mechanics make his chances an arguable 50/50. On to Cloud Strife, the other non ballot third party DLC. I don't get why people think he's destined to never come back. Some say Cloud only got in 'cause of Sakurai's generosity, but it turned out that Sakurai actually asked Tetsuya Nomura first and not the other way around. Plus, Square Enix dosen't really have public image problems like Konami does, so they would be more willing to arrange for Cloud's return (I can't find anything related to it, but I could have sworn I heard that Tetsuya or someone else said they'd be open to Cloud appearing again) And finally Bayonetta.... hooh boy, Bayonetta. To those still sore over her total dominance soon after her release, I've got bad news. She probably won't be leaving. A: Like it or not, she still won the ballot, giving her a reason to be prioritized, B: Nintendo practically owns her nowadays, with Bayonetta 2 being funded by Nintendo, and unless I'm mistaken, I hear Bayonetta 3 is releasing exclusively on the Switch, and C: Platinum Games and Nintendo have a solid relationship, and Hideki himself has stated he's fully open to Bayonetta's return.
.....Damn, that was one long post.
