I dunno that that really means anything since Jigglypuff is usually unlockable and as a result typically gets put in with those characters. For reference, Falco and Wario were also in the "Secret Fighters Unveiled" section of the Smash 4 Dojo, and nobody ever says a character like Wario was low priority.
As for who you would prioritize underneath Jigglypuff and Lucas in a future game, we really have no idea since we don't know how that works, but Puff may get a lot of leeway just for being part of the original 12. Priorities can change over time if time becomes short and Jigglypuff may have a fast track to the top when time starts to run out because of its status. We really have no way to know, but it hasn't missed a game in 25 years, so it seems a bit silly imo to pretend it might have a shot of missing out on a game.
It's not hard evidence, as I stated, and I realize people may put different weight into it as an indicator. However, basically every character in that boat is 100% believable as a lower-priority character. You point out Wario, but he's like the exception that proves the rule; maybe there was some issue going on with him that we don't know about, or maybe he was just outranked by the rest of the Mario cast and so viewed as less essential than fans would think, idk; in any case it's really only one character to explain away. You could say Ganondorf as well, though he is still a derivative so it fits with being added later in development nonetheless. Same for Falco, but I actually think he was probably gonna be on the lower end regardless, as a sidekick-type from a midsize series.
On the other side of the coin, I think when cuts are likely to be more prevalent than ever, it's more risky to pretend it
doesn't have a shot of missing out. I seriously doubt they would dramatically change a character's priority mid-development; if time is getting short, it's possible an easier-to-make character could jump the line a bit, sure, but that would only be a matter of one low-priority character getting in over another low-priority character, not a low-priority character suddenly being high-priority.
As for Puff's original 12 status, I personally wouldn't put too much weight into it. The original 12 is extremely lucky even to make it to this point without being touched. Ness was straight-up planned to be replaced in Melee until that didn't work out, Jigglypuff is confirmed to have been low-priority at least in Brawl, and who knows what would've happened in Smash 5 without EiH. All this tells me that such a thing would only be a minor consideration at best. When it comes down to it, they can't afford to give arbitrary boosts to characters like that if they're trying to put together the best roster possible.
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Uh, no, Jigglypuff still is easier to make than Wolf was. Because it was in earlier games(a key factor that means it's far easier to bring the Puff back in the first place). There's really nothing to suggest it was at all lower priority outside of Brawl, but that was a circumstance that never came up again too. And we don't know the exact context. It may not have actually been lower priority, just done last because it was easier to make. What is clear is that it wasn't planned as an important piece of the SSE, but that's all we can directly get from it. We're solely basing that on loose information with far less context than what is needed.
My bad on the Wolf bit... which further shows my point of how much harder he is to make than Jigglypuff anyway. Since it takes less effort due to both having a previous build and Kirby to help speed up the model stuff. Every single character requires balancing, etc. due to having any difference whatsoever. That's a given. Absolutely no character who is added separately would take the exact same effort(not even Daisy, who definitely has the lowest work done balance-wise out of any character, respectively).
The fact it was on the original project plan and made it into the final game already makes it clear it's not as low priority as we actually thought. Mewtwo, Dr. Mario, and Roy were the only low priority veterans specifically that had any work done. Young Link and Pichu were the only ones outright low priority due to not even attempted to be brought back(to what we have data on, anyway). Dixie couldn't get far enough in development, so is our only low priority newcomer we know of that got anywhere. And the other Forbidden 7 would be the last 3, respectively(Toon Zelda, Toon Sheik, whatever Pra_Mai actually is).
I never made any specific point about Wolf, other than simply pointing out the inaccuracy with the 70% quote. Anyway I can turn it around and say that even if Jigglypuff were about as easy to make as a clone character, that can still be a lot of effort and definitely not a simple throw-in addition.
And I know the balancing has to be done with any character; that was exactly part of my point, that balancing Jiggs's unique moveset is still a time/resource factor even if you ignore all the effort that goes into modelling and animating it.
Finally, yes, we do know for sure that Jigglypuff was low priority in Brawl. See here:
https://sourcegaming.info/2015/08/17/sakuraibrawlquestions/
He straight-up says that the three not involved in the SSE plot (Wolf, TL, and Jiggs) were on the brink of being cut, and that they may not have made it if they didn't have as much time.