I haven't really voiced my support for Rex before in fear of XC2 spoilers, but I guess now's as good a time as any to do so. I love XC2 and would really love to see Rex & Pyra make it in as a duo. I have an idea on how they could function, too. You would control Rex, using the Aegis sword to attack and whatnot, while Pyra follows closely behind. Pyra acts as your support, and the affinity line connecting them is visible. If you stay close to her in battle, your affinity will gradually increase, strengthening Rex's attacks. If you stray too far from her your affinity breaks, leaving you weaker until you reconnect the bond. Like in XC2, Pyra can't actually be attacked, and if she falls off the map she will respawn on the stage as soon as Rex makes it back to the ground.
That would be their main gimmick, but I had a couple of other ideas I wanted to share. A few moves I'd like to see included in Rex's arsenal are Anchor Shot either as his grab or as a command grab that trips the opponent, Double Spinning Edge as side special (having the potential to break shields?), the basic auto-attack combo animation for jab, Burning Sword as their recovery move, and a chain attack with Nia, Dromarch, Tora, and Poppi for their Final Smash. This next idea is a little gimmicky and might be unreasonable to expect, but it would be cool if you could cancel some of your normal attacks into specials just like in XC2. For example, cancelling the third hit of your jab into Double Spinning Edge. It would make battles feel a little more authentic in my mind, and it would open up the opportunity for combos you otherwise wouldn't have.
Regarding alternate costumes, I think it would be a really smart move to allow the player to choose between Pyra or Mythra from the character select screen, just like choosing alts separately for the Pokemon Trainer and his/her Pokemon. It would be nice to be able to swap between them in battle, but without elemental weaknesses I feel like there wouldn't be much of a point in a mechanic like this since their abilities are so similar. This way, fans of Mythra can be happy too without needing to integrate a blade swapping mechanic into the moveset.
Our situation for DLC is indeed strange, as plenty of you have pointed out. The way Sakurai phrased things during the Direct really seemed ambiguous as to whether Rex has a chance or not. Regardless, I'm still going to hold out hope, so let's look to the future together, shall we?