After thinking on it a while, I wrote an argument as to why Elma has a really good chance. I am not going to expect Elma, but I will have hope.
1.
Sakurai probably does like Xenoblade Chronicles X.
Sakurai is a big fan of the first Xenoblade, so I really think he would've at least tried Xenoblade X in 2015. And, well, I just can't imagine him having the same reaction as those who hated the game because of its main story and/or music. None of Sakurai's games, besides maybe Kid Icarus Uprising and Brawl, have a big focus on story. He's always put gameplay first. He is a fan of FE Awakening and Fates despite many people hating the story in those games. Sakurai was also good friends with Iwata so I'd think he would've seen the Iwata Asks with Tetsuya Takahashi (or any of Takahashi's interviews about Xenoblade X) where he explains that the focus was the world, gameplay, and side quests which they put an incredible amount of heart and effort into. As a game developer himself, Sakurai must be able to appreciate that. In his Famitsu column on Xenoblade 2, he praised the open world a lot and mentioned specifically the "sense of freedom"... It's easy to argue Xenoblade X has a better open world with a bigger sense of freedom.
2.
Xenoblade Chronicles X DLC would be a good business move.
I've been told there's little chance of X content being DLC because the game didn't sell well. Except, despite being on the Wii U, it sold better than most Fire Emblem games including Path of Radiance, Radiant Dawn, and Shadows of Valentia. The sales weren't that great, but that could also make Sakurai want to represent it to increase its popularity. Xenoblade X will still most likely get a port (and a sequel!) eventually. Monolith Soft and many others are confident in the game's quality. More people just need to be enticed to try it. Representing X in a significant way would definitely help the sales of the X series moving forward.
3.
There's practically no excuse for having no Xenoblade Chronicles X music.
Hiroyuki Sawano is very famous in Japan. I'd think that even if Sakurai personally isn't a fan, he'd see if some music from Xenoblade X could be included considering Sawano's popularity. I don't know too much about Sawano himself, but he certainly doesn't seem too strict about the use of his music. Most of it can be found on YouTube. Monolith Soft got it in Xenoblade 2. Studio Trigger got music he composed for Kill la Kill into Space Patrol Luluco. If his music isn't among the locked songs in Smash Ultimate, either Sakurai doesn't think the style fits Smash at all AND none of the composers for the game could/wanted to remix any of the songs to better fit Smash, or it's being held back for DLC.
4.
Xenoblade Chronicles X deserves a stage.
Mira is perhaps the most impressive open world Nintendo has ever made (alongside BOTW). Exploring X's world is basically the point of the game. There are tons of beautiful set pieces; just look again at the exploration trailer!
https://youtu.be/AvpNXhzscjQ?t=38 A place like this would be a more exciting addition for most people than something like a 6th Kirby stage for Bandana Dee. Between Smash 4's DLC and Smash Ultimate, the only new stages not tied to a character from an unrepresented series are Super Mario Maker, New Donk City Hall, and Great Plateau Tower. Not many at all... It's possible Sakurai wanted to have a stage set on Mira, but couldn't because of time constraints--I imagine it being more work than one of those three if the stage takes place at multiple locations.
5.
Elma is one of the best choices still in the running to be playable.
Most people still seem to be predicting Bandana Dee as Nintendo's DLC character (assuming the other four are from 3rd parties). While a spear user would be cool, there's not really that much you can do with a spear. Throw, thrust, swing, spin...that's about it. Bandana Dee himself doesn't have any cool abilities. I think the Belmonts, Corrin, and Ridley (to some extent, with his spear-like tail) have cooler aspects to their long range movesets than Bandana Dee could have. Elma would be unique because in addition to swapping between dual blades and dual guns, she has Shadow Strike (like Shulk's back slash, but probably as Up Special with a high jump), Sliding Slinger (low-slide while shooting forward), Ghost Factory (which might, for a limited time, make the next single attack that would've hit miss) and Tension Points/Overdrive, which could make her moves more powerful. We've seen how important a unique moveset is to Sakurai when choosing newcomers.
