Well, I wouldn't say headcanon. I've only ever called it a theory, because it made sense from a card ranking perspective. It should exist technically
That said, I still think its possible with Yamato, being she is Kaido's child. Labeling her the Ace would just make sense, even if she's not the strongest. Anyhow, while they are virtually fodder, we still haven't seen 5 Numbers still, so i wouldn't find it at all shocking if the Ace was introduced as an Act 3 closer to show how hopeless everything is, but I personally doubt it
Edit: Its headcannon to assume its Kaido or Yamato someone new. Its a theory to "think" its either of them or someone else.