The evidence isn't pointing towards anyone right now, but I can at least explain why KOS-MOS would be interesting and a viable Bandai Namco candidate. Most of us are expecting at least one more Bandai Namco rep, because of their impact on the industry, very close relationship with Nintendo, and co-development role in Smash Ultimate.
With Bandai Namco's main mascot already in Smash, it's possible that they would opt for a less anticipated, yet still hype-inducing pick like KOS-MOS. I mention this because Bandai Namco crossovers are pretty ridiculous. Soul Calibur's Astaroth was put in a Namco party game, Spawn and Star Wars was in Soul Calibur, Negan and Gon are Tekken characters, etc. They just... don't care. If it seems like a cool idea, they'll most likely roll with it.
With that being said, KOS-MOS is one of THE robot girl in video games; it's an archetype that hasn't been explored yet in Smash. Next, keep in mind that Bandai also make toys. And they've made many KOS-MOS figurines that sell like hotcakes overseas. So, if she had Amiibos, they'd also probably sell effortlessly and in large quantities. I can imagine her selling way more than Heihachi, for example.
Xenoblade (Nintendo) is the spiritual successor to Xenosaga (Bandai Namco), which is where KOS-MOS originates from. KOS-MOS is the face of the franchise. Monolith Soft put her front and center on their
20th Anniversary Commemorative Website. (Fun fact: Naughty Dog couldn't even put Crash Bandicoot on their anniversary celebration because of Activision. I mentioned this to give you an idea of how close and interchangeable she is with both companies involved).
Out of any Xeno character, including Shulk, KOS-MOS is used in crossovers/promotional materials the most. Both Bandai Namco and Monolith Soft love her, and - design, personality, plot, or some combination of all of the above - she's the inspiration for many of the characters we know and love today like Fiora and Elma. I mention all of this because quite a few folks would like another Xenoblade rep. Due to time conversions, we couldn't get Rex and Pyra, but it's possible that Monolith (with the approval of Bandai Namco) put KOS-MOS on the table.
Interestingly enough, some of her cameos are implied as canon appearances because of interdimensional travel shenanigans. Thus, she's lived on past her initial PS2 appearances.
Now, let's list her cameos. She's been in Namco x Capcom, Tales of Hearts, Super Robot Taisen, Project X Zones 1 and 2, and Xenoblade Chronicles 2. She's also been a costume in Soul Calibur 3, Tales of the World Nakiri Dungeon 3, and Tales of Vesperia.
I can imagine Bandai Namco choosing her, especially if they make a Xenosaga HD remake or something. More people are interested in the character, especially after her Xenoblade cameo. It's true that Harada recently stated that the series isn't marketable. However, one way to increase interest in a character? Putting them in Smash.
Now to explain why she'd be a fun pick: the moveset potential. KOS-MOS is a self-classified annihilation weapon, so her abilities are totally bonkers. She has handguns, lasers, hammers, bows, drills, energy wings, gatling guns, shoulder mounted rail cannons, etc. Not only can she morph parts of her body into them because of nanomachines, but she can also summon them from literal hammerspace, and then have more fitted on to her. There's a reason people buy her aforementioned
figurines like hotcakes (just look at all of those accessories! That's not even half of it) because she's a one woman army. It's easier to show than tell, so take a look yourself:
Tl;dr: It might not happen, but it's still possible and a good idea. The decision wouldn't be out of character for Bandai Namco. They could use the opportunity to sell merch. It'd also satisfy many Xenoblade fans, while Smash simultaneously gets the benefit of adding a new franchise. It could promote a series with a dedicated fanbase that have been begging for HD ports, it would bring a new archetype to Smash, and probably one of the more ridiculous ones because she's basically Superwoman (complete with an endless list of weapons and abilities). Now... here's a more detailed explanation (some of this is new, so I didn't completely copy/paste my old response).