All right, lengthy post, but these are what I consider to have the highest chance of being the DLC characters, based on what I think Nintendo would select for Sakurai.
1. Fire Emblem character
Come on, you all should know already how much this series loves this franchise. Now as for who, I have 3 theoretical characters:
-Azura
Wether you like it or hate it, Fates is the best selling game of the franchise, and along Awakening, has the most exposure. Azura is the deuteragonist of the game, so I could see her getting her way in. That said, she’s the one I would expect the least for a couple of reasons, first being that by this point, a following game in her series was released and a new one has been shown and scheduled for release in just two months, so she’s not so relevant. And second, she’s shown in the website to be a spirit, which I am not 100% sure it can deconfirm anyone since she’s not a “Fighter Spirit” but I’ll keep it in mind.
-Alm &/or Celica
The two of them are relevant due to the latest game yet before Three Houses being a remake from theirs, not just that, but Celica is particularly popular out of the two and having a decent fanbase that want her in (including me). Of course, popularity isn’t everything when it comes to Smash, but with Ultimate being meant to be fanservice: the game, and K. Rool and Ridley being included because of that, can see one of two being one of the selections Nintendo made. Should be noted both are a single spirit as children tho, but like Azura, not sure if 100% deconfirmation.
-Three Houses MC
The most likely in my opinion. I don’t see Nintendo in any way not using Ultimate to promote new games that have been scheduled for release recently. Not just that, Smash has had a history of pulling a FE MC to promote the most recent game at the time, it happened with Roy, with Ike and then Corrin. Many people have been calling on Edelgard, but...I think they’ll pick blue haired protagonist over her.
2. Octolings
This is a pretty obvious one and I am surprised that they aren’t in the base game as Echo Fighters. Splatoon is easily the most popular recent IP that Nintendo has had in years and is obvious it’s gonna continue being one of their main franchises for a while. So, why not include a second representative in the form of the Inklings worst enemies?
Well, I guess that because the draft for Ultimate was done by the time the first game was the one being played and the Octo Expansion didn’t exist, maybe Sakurai didn’t see them as a worthy addition. But now they have exploded in popularity thanks to the Octo Expansion, so I can see Nintendo’s marketing department wanting them in, even if they’re shown as a spirit, which again, not sure if 100% deconfirmation.
3. A third party character
Could be one, could be two, but I can see that one of the third parties involved would want a bigger piece of the cake. Heihachi is a popular option, moreso since Sakurai actually considered him for 3DS/Wii U and Namco hasn’t been shy in lending their properties as co-developers of the game. But Heihachi is still just one of multiple possibilities. I can see for example Jeanne being made a fighter to promote Bayonetta 3 (and yes, once again, I know she’s a spirit) or perhaps, who knows, Noctis due to how SE still loves milking his game to death asking Nintendo to be a selection.
3. Bandana Waddle Dee
Okay, okay, so...This one is actually a more personally wanted from mine. I know many people are asking for Marx and Magolor, and that’s okay, they do have a shot. But Bandana Waddle Dee is simply more prominent than them, and I have a feeling that’s partly what Nintendo would want when selecting characters, that they would be prominent.
Bandana Dee by this point, has become a staple and a main character for the Kirby franchise, appearing in multiple games often in an important role to help Kirby, not to mention he’s a highly requested and recognizable character that is easily marketable. Can see Nintendo placing him on the suggestions with little to no doubt.
5. Xenoblade character
I’ll be honest here, I barely know anything about Xenoblade. Would buy the first one if I had any money...Or the 3DS port if mine wasn’t an original model 3DS...But back to topic, following Splatoon, it seems to be the second most succesful recent IP Nintendo has had, and Sakurai was well aware that Rex was a highly requested character, so must Nintendo too. Xenoblade 2 was a commercial success, so a new one could be in the works as we speak.
The way I see it, we could have Fiora (Chrom is still in Robin’s FS, so saying a character is deconfirmed because of that is out the window), Elma (unlikely since X has lost relevancy), Sakurai changes his mind about Rex, or a new entry gets announced and its new MC is included.