I've been toying with a few ideas for the protagonist of Xenoblade Chronicles 2, Rex. I'll simply start by flat out saying there's not much there for Rex to be a clone of Shulk, but rather a character with features of his own. He'll be a great addition if you ask me!
For those of you who haven't played XC2, Rex is what they call a "Driver". The driver is usually a sword-wielder who is the lead in a partnership with a Blade. A Blade is effectively the spirit of the weapon, which can manifest itself in some sort of form, whether it be human or animal form. Rex's main blade is Pyra. Pyra, as the name implies, carries fire elemental power in her attacks, and is effectively tied to the weapon that Rex carries: the Aegis. Because he is the wielder of that blade, Rex is called the Driver of the Aegis in-game.
One major part of the game (like, practically the entire story) is based on the growing relationship, trust, and love between Rex and Pyra. This also shows itself within the gameplay of XC2. Because of this, I don't just advocate for Rex to be in Smash on his own. Actually, I advocate for Rex and Pyra to be some sort of partner character, but not like the Ice Climbers exactly.
In XC2, drivers and blades work together to defeat enemies. The bond between them is displayed by a faint blue line that connects them...it's basically the energy between the characters. The closer they are for the longer period of time, the more powerful it gets. When it's at it's peak, it turns gold, and allows Rex to charge his arts, and therefore use them, more frequently and at a higher level.
Therefore, I advocate for the duo to work in one of a few ways:
1: Put Pyra at the center of the stage similar to Pokemon Trainer with the affinity gauge growing as Rex stays close enough. If he gets knocked back, it gets weaked. The longer he has it, and the more damage he applies to his opponents, the stronger arts (or specials) he can use. In Smash, the duo could be dominant if not countered, or dominated if not careful.
2: Make Rex and Pyra almost like a team in MvC. What I mean by this, is that Pyra (or Rex if switched out) can jump in briefly for a combo attack to continue a working combo or a combo break. This could be tied to a particular special attack for the character.
I'm not entirely done brainstorming about the character, but these are two ideas. There are some uh,...spoilery things I would feel bad talking about that could be used in the moveset, so I don't want to discuss that yet. I'll put it in spoilers.
EDIT: Phew, almost screwed that up. I'll leave this for now because I don't want to spoil anything for anyone. I'd love to hear some ideas!