Topic: I have not really explored it until now, but I think I figured what would make Shulk, Elma, and Rex distinct. Shulk is basically a character who can change his stats through modes via neutral special. Elma could change weapons via neutral special for different set of aerial/standard attacks while stats for specials and Smash attacks remain the same. While Rex would be able to change his Blade via neutral for a different set of specials and adjusted stats. Do you feel all three are distinguishing enough to warrant all 3 in the game? I need opinions on the matter.
I'm a huge fan of Xenoblade so I'll answer as best I can.
Elma isn't really comparable to anyone currently on the roster, much less Rex and Shulk. She's much faster than them and fights with dual short swords and dual guns, two weapon types not in Smash. Her moveset would most likely center around various multihit strikes with her swords and various gun streams, with a few Art references to stuff like Executioner or Hundred Shells in the normals.
As for her specials, they'd be oriented around positioning to emulate the Full Metal Jaguar class's strengths. Shulk has Back Slash, but Elma would take that to a whole other level. Something like Shadowstrike would function just like Back Slash, but there's so much more that can be done with it. Ghost Factory could function solely as a movement option/recovery move, Shadowrunner could apply a back attack bonus for a short while and maybe have a dash effect, Sliding Slinger could expose an enemy's back when she crosses them up, and a few of her normals like Hundred Shells could also have some of these secondary effects.
I guess Elma could have a stance mechanic of sorts but it makes a lot more sense for her to just use a 50/50 split of sword and gun moves to avoid overly complicating things. I mean, in X your equipped Arts stay the same regardless of the weapon last used. It's not like you get 8 gun arts and 8 sword arts that can only be accessed when you have the corresponding weapon equipped. The only move that changes is the auto attack, so if you really wanted to reflect this only the jab would change depending on what move she used last.
All of this adds up to a completely unique character that warrants an inclusion.
While at first glance Rex may seem a bit similar to Shulk, they really don't have anything in common aside from having a giant sword, and even that comes with its fair share of differences. While the Monado's beam is a sourspot that can even be delayed upon activation (Forward/Up Smash), the Aegis Sword doesn't have to function the same way. If anything, the extended beam would deal fire/light damage instead of being a sourspot. And while Shulk swings his sword relatively effortlessly considering how huge it is, Rex's weighty strikes are entirely different. He'd be much stronger than Shulk at the cost of mobility. Seriously, look how slow he runs with his sword out. I guess he'd also be a fast faller if him being a salvager is anything to go off of, contrasting Shulk's average base stats that are probably that way because of his Monado Arts. And Shulk's standout mechanic, the Monado Arts's stat mods, wouldn't be implemented with Rex unless you committed to a complicated and weird weapon switch system in which case Rex would be entirely different and this debate wouldn't exist.
I haven't even mentioned how aesthetically different a lot of Rex's attacks are. He has fire/lightbending feats, whether it be through spinning flame pillars or projectiles, and 3 of his Arts, Rolling Smash, Anchor Shot, and Double Spinning Edge, are very unique looking in appearance and/or function.
Then you have Blades, which bring with them two key things: an affinity link and Blade Specials. The affinity link would be a different take on Lucario's Aura, as a movement and other potential buffs would be granted at max affinity, but affinity would be tied to keeping Rex close to his Blade instead of getting damaged and therefore knocked away from them. In my moveset I have Pyra transforming into Mythra at max affinity to further indicate the buffs and bring with her elemental and Blade Special changes. Blade Specials on the other hand are a different take on a duo character. There's only one weapon between the two fighters, so one input only equates to a single move instead of two. I have Blade Specials 1-3 as Smash Attacks and level 4 as the Final Smash. There could be more weapon passes in the normals to further differentiate Rex from Rosalina and ICs, but they wouldn't be based off of anything in particular.
There's a whole different question as to how Blades would actually appear. Whether or not they walk on the stage behind Rex (my personal preference) or run around the background and only pop out for Blade Specials, that's up to you. But either way Rex stands out from the crowd.
After thinking about it for a bit, I personally don't think we'll get Rex in the base roster. I'm just not sure if Sakurai even knew of him back in 2016 and even if he did, whether or not he'd feel he could faithfully portray Rex without playing his game. Rex & Pyra are a good bet for DLC though. So in the end I do think we'll get all 3 of Xenoblade's protagonists, but not at release.
