Very much like our good friend Isaac, who conspicuously is rarely included on lists like that despite just as frequently being among the most popular, and at one time or another being more popular than any of the four. Around Dark Dawn he was the most requested character. During the ballot he was somewhere in the top three. After the August Direct until the November Direct he was probably again in the top three. And his popularity dates back to the initial GS games.
And even if he's not right at the top, for big chunks of the last decade he's floated around in the top ten.
Never seems to get the nod, though.
I have to wonder if Sakurai's kinda stumbling over Psyenergy when trying to imagine how he'd work? Or running into potential technical issues (esp. considering the Rex / Pyra idea was scrapped)? I'm completely spitballing here, but still.
The first is not a thought I'm entirely confident in since that could easily be managed by shrinking the hand size or using other attacks in his arsenal (I've never played a GS game to save my life) but Psyenergy kinda strikes me as... indirect? It doesn't follow usual attack conventions in Smash - the only remotely similar ideas I can think of are Bayo's smashes and Steve's D-Throw. Doesn't mean it can't work, but Sakurai's had previous stumbles imagining how a full fledged character fights -
and
.
In any case, not only could Sakurai gain a solid idea later on, but future Smash games are going to be presumably on stronger hardware. So that might help Isaac's situation down the line.
Lucario does have a pretty polarizing game design.
I don't think Lucario is polarizing so much as he's annoying to everyone (except for maybe characters that tend to K.O. off of stray hits), which is arguably worse.
Volatile might be a better word because of how knockback is heavily influenced by Aura -
base % is 65 % - and it makes any situation where he's above say 110 % extremely dangerous for the opponent - it makes him also very difficult to balance. At the same time, Ultimate has no shortage of characters who can kill at that 110 % mark and his weight nerf from 4 impacted him pretty badly (plus he has lingering inconsistency issues). Still though, I kinda think he's not far from being a more consistent character: he IMHO needs primarily consistency fixes, specifically on his multihits. Maybe slight improvements to his frame data... although I'm not sure on that.
What first party characters seem plausible, if the final reveal winds up being one?
Cinderace seems like a front-runner, but beyond that, I have no idea. (an Advance Wars remake is coming out, and I'd be interested to see what they could do for Andy or Sami, but I doubt they'll add an Advance Wars character)
Try as I might, and as much as I'd love to see them playable, I can't see Nintendo ending off DLC with Dixie Kong, Captain Toad, or Bandanna Waddle Dee, since generally speaking, it seems like they try to focus on new-ish releases when it comes to first party DLC newcomers.
It's a really difficult question because I'm struggling to find a clear frontrunner that doesn't come with a caveat attached to them: I'd say that one of Pokemon, Chozo General / EMMI, Octoling and Ayumi would be closest to that, but I'm not sure if they were really in serious contention by the time FP2 was finalized. The Sword / Shield spirit event doesn't really help matters due to its timing being very close to FP2 getting locked in.
I'm still settling on Ayumi, even as I'm not sure on her either - IIRC the FDC remake wasn't initially planned to get an English translation.