So I was thinking of second chance theory and I'm not sure if this was discussed before, but I think I found a new pattern. So I have a new theory which is an extension of second chance theory and I called it "too complex for being in the base game theory" (yeah I couldn't come up with a better name). The thing I realised with fighter pass 2 is that a lot of the characters were problematic for Sakurai and the team.
-Min min: Sakurai said that he had a hard time balancing her considering her huge range.
-Steve: Sakurai said that it was a huge challenge for the developpers to change every stage so block can be placed on them
-Sephiroth: unsure about this one, but I think Sakurai wanted to make feel Sephiroth like you're playing a boss, without him being over power
-Pyra/Mythra: Sakurai said that he wanted Rex + Pyra and Mythra, but Rex design was way too complex to be there all the time and work with 8 player smash, so he had to cut him. This mean that while Pyra/Mythra themselves are not overly complex, Sakurai did try something with Rex which was too hard and just didn't work
-Kazuya: Sakurai considered a Tekken character in Smash 4 but he thought it would be too hard to implement their movement.
So my theory is that second chance theory is not just giving a second to some random character, it's giving a second chance to character that Sakurai couldn't implement in the past. My guess is this might be why fighter pass 2 is taking much longer than pass 1, he knew that those characters would take a long time to work with.
So with that in mind, as much as I doubt it, the first character that come to mind is Chorus kid. While Sakurai never talked about them, it was often speculate that the reason they might have been cut was because either the trio character was too hard to work with, or he was trying to implement some sort of rhythm mechanics which was too hard to work with too. And now that with dlc he have way more time to work with a single character and put the whole team into it, now might be the time to reuse this concept.