I haven't been able to respond to this thread as much as I would have liked the last couple of days, but here are some notes on a few recent points of discussion:
Hopefully this response isn't moot now since it's been so long, but:
For Impa, I would expect the Ocarina of Time design to be more likely than something from spinoffs like the Warriors design, but they might also give her a hybrid, modernized design if she gets in.
And yeah I agree that something from Legends Arceus is much less likely than something from the newest main series games, but if they were gonna put in a Sinnoh mon it would make more sense now than it has in a while, which I think was what I mostly meant.
I'm not as avid a speculator on third parties, but since you asked, I'll comment on those in your updated roster. Bomberman is definitely possible but I think he's mostly riding on pure demand. Arle and Zero I think are less likely than other characters from their companies like another Sonic rep or reps from Monster Hunter or RE, but not total left-field picks. Arle's chances in particular basically rest on Japanese ballot support in my estimation. Namco DLC is likely. Hayabusa and Eggman could also be candidates for base game as retro-era 3P reps, and both have realistic chances. Iori and Crono I think are reaches but I'm guessing you're adding them as "dark horse" candidates.
Ah, I'm not too familiar with design differences since I assumed that HW Impa was just her Ocarina design through and through. I do agree about the possibility of a "hybrid" design for her, though alas, I'm no artist, so I can't really imagine how it'd look like exactly.
I get what you mean by a Sinnoh mon. I still think a Gen 9 or 10 mon will be TPC's desire whenever Smash 6's project plan is being worked out, but if Sakurai was able to assume total control here and pick a popular Pokemon choice instead of a "promotional" choice, I think Cynthia would work as a Pokemon Trainer system character, considering her popularity among the franchise's cast of human characters. The Hisui Pokemon Trainer could also possibly work in that regard, especially since it would give Sakurai a chance to revisit having Decidueye as a fighter (even if it isn't the normal Alolan version of it).
Yeah, Bomberman is mainly driven by his demand at this point, which is why I dropped him in base game where the picks are more likely to prioritize that kind of stuff. Though admittedly, besides demand, it's hard to think of Konami/Hudson properties that would take priority over Bomberman now, besides if they add Raiden or Alucard into the game as second characters for their series. Contra and Gradius are both options I've humored (and I'd be up for the latter), but the protagonists from those games would probably be rather difficult to implement a practical moveset, both being more projectile-heavy than even someone like Mega Man, to the point where neither of them have any real close combat options in their source games.
I think that in the end, both Arle and a second Sonic character will be added to the next Smash, though I will admit that it's rather hard for me to completely decide which one would be prioritized. On one hand, Sonic as a franchise hasn't gotten much proper growth in the last Smash installments, which indicates to me some sort of hesitation to give it attention, but on the other hand, Arle is definitely weighted towards Japanese demand, even if it's been five years since Puyo games started getting consistent localizations worldwide. It's hard to say which one would be focused on being added first, but I think it will ultimately hinge on who would be easier to develop. If the Sonic rep is someone like Knuckles or Shadow who could easily use Sonic's frame (and in the case of the former, already has some fighter-related code thanks to his Assist Trophy), then I could see them being base game and Arle being a DLC fighter. If it's Eggman, who I imagine would be a much more complex character to develop, I think he'll be in the DLC lineup instead.
I'm hard to budge when it comes to Zero, mainly since, like with Knuckles above, his Assist Trophy is also pretty fighter-like already, but just because he's the only Capcom character on my roster doesn't mean I'm ruling out other characters here. I feel that Dante, one of the MH monsters, or Phoenix Wright all have a fair shot as an extra DLC fighter (I only had room for 9 DLC newcomers on my image, compared to Ultimate having 11 DLC newcomers, 10 being planned from the start. If I had room for just one more slot, I'd probably put something Capcom-related there.)
I actually considered Hayabusa as a base game character, but considering the number of returning third-parties already in the roster, I feel that it'd be easier on the team to wait until DLC before trying to license content from Koei-Tecmo, just so it wouldn't add onto the base game workload of relicensing everything from the other companies as well. For comparison, I feel that the Belmonts got into Ultimate's base roster mainly because it was more or less a two-for-one deal with Konami, since Snake also came back after missing a game.
Yeah, Iori and Crono are definitely dark horses here, though I do have my reasons, the former being a character Sakurai has explicitly expressed his liking of when talking about SNK's games in the Terry presentation, and the latter hailing from a game that many consider to be a cult classic among JRPGs and probably the most popular of Square's smaller JRPG outings (since I doubt Dragon Quest will get a second character, and I especially doubt that FF7 will get a third character when I'm also predicting that Sephiroth will be packaged as veteran DLC again.) I think the dark horse description hits the nail on the head quite nicely, though; not many people pay attention to these characters, but the ones who do, have strong reasons to do so.