I honestly have no idea what they could choose for the Legend spirit (and its fight). I'll spit some ideas:
"Boss" rush against Torna: similar thing that they did for Persona and Dragon Quest, except with antagonists this time. The challenge mode in Xenoblade 2 already features a battle that is kinda like this.
"Boss" rush against party members: same thing but with the good guys. This one is more lame though because all of them already have fights in the base game.
Jin and Malos: Easily one of the most memorable boss fights in the game, against two fantastic antagonists. One of the hardest ones too. This fight is so ****ing hard that the victory condition is simply dropping the health of one of them to half (not both of them, just one) and a lot of players still lose it quite a few times in their first playthrough, myself included. There is a challenge mode version of this battle where the challenge is simply that you have to play the fight properly all the way to the end.
Malos or Jin on their own: not like they're pushovers alone. Although I do feel like they lose some flavor AND it would require putting one above the other. Essentially superpowered versions of Sephiroth for Jin or Shulk for Malos. I feel the Aegis, Malos is the most likely of the two because of his relationship with Pyra and Mythra, themselves already legendary spirits.
Final boss of chapter 9: it's a spoiler, but it's referenced in their trailer so they might go for it. The Acting God, Amalthus.
Coffee with Milk: another spoiler, and they would have to work around some technical issues, but it also seems like something that could be legend spirit material.
Siren: this one seems hard. I don't think any character in particular resembles Siren even if you made them giant and metal, and even if you went for something like Galleom, he looks so goofy compared to Siren. Still though, this giant robot is ever present in both Xenoblade 2 and Torna.
KOS-MOS: The rarest and quite possibly strongest and most popular of the rare blades, this DPS/Tank/Healer has received quite a bit of attention. With an exclusive Xenoblade 2 design and the privilege of getting an official figure before most of Xenoblade 2's actual main cast (and don't tell me they don't pay attention to those things when Pyra and Mythra's trailer directly references their own official figures), they could possibly get away with putting her here. According to her own words: "her power output is on par with Mythra's", who is a Legend spirit herself. She's a logic driven android so you know she isn't just bragging... is she?