Do want to point out that Season 2 having “no bearing on base game spirits” assumes Spirits will be chosen in the first place as opposed to each fighter pass being essentially entirely new to Smash. This is consistent with every challenger pack save for Corrin and Byleth, both main characters from recent Nintendo games.
I’m not attempting to start another Spirit debate, but we can’t possibly know that any particular Spirits are “fair game” for Season 2 until it actually happens.
For all the things that are called “fan rules”, that is also one of them. I wouldn’t call it impossible. But for now, we literally cannot know if Spirits actually have a shot, and we won’t know until it actually happens.