I started the game not liking Rex much (I preferred to take care of surviving, so I mostly used Nia), but as the story progresses, I've come to find him a much more interesting character (naive but learning, hopeful and determined). Pyra/Mythra too had much more depth than I initially expected. I'd more than support Rex/Pyra for Smash!
I was thinking of what their gimmick could be... and there's just quite a few features from XC2 that would be interesting in smash.
- There's normal attacks, Driver arts, and Blade arts.
- Normal attacks and Driver arts can be cancelled into Driver or Blade arts.
- There's Driver combos and Blade combos.
- Blade combos seal a power from the enemies.
- You can change blades to change element and moveset.
Here's the "coolest" but unlikely moveset I can think of;
Normals, Driver arts and Blade change can be cancelled into Driver arts, Blade arts and Blade change, which cancels the endlag of the move, maybe only if they hit an enemy. All moves have relatively longer endlag compared to other characters. Driver arts need one or two normal attacks to recharge, Blade arts charge with two or three cancelled-into driver arts.
Normal attacks: jabs, tilts, smashes and aerials (except Dair).
Driver arts: Side B Rolling smash, Up B Double Spinning Edge (a bit like Link's up B), Grab/Tether Grab Anchor shot (If cancelled-into, "trips" instead of grabbing, which could be teched like Ganandorf's Side B. This would represent "topple"), Down Air would represent "smash". Up throw could represent "lauch" (after a topple).
Blade Switch: Down B. Could simply change the element of the blade rather than the whole moveset.
Blade arts: Neutral B. Rex returns the weapon to the Blade and they execute the Blade art corresponding to the amount of charges they have. Meanwhile, Rex receives a massive mobility bonus, but can't do any attacks (in this state he can only move, jump, shield/dodge, and tether recover). Thus Blade arts could be used as a sideways recovery move. Landing a blade combo on an enemy "seals" something depending on the last element. E.g. lowers their knockback, damage, etc. A bit like reverse-monado arts.
Final smash should be bringing Nia and Tora for one of those simultaneous attacks that destroy bosses. Other things could be even better, but it would come into spoilers.
Alternatively, especially if changing blades is not implemented... Then different blades (especially Pyra/Mythra) could be parts of the alternate costumes...