This brings up the important question of how many and/or which characters had their priority boosted to accommodate EiH.
I assume most people would point first to the Melee clones and other characters who have been cut before, and yeah many of those probably would not have been included if EiH wasn't deemed achievable. But there are also questions about how hard they would have tried and/or how worth the budget space it would have been to re-include certain characters like Cloud or potentially Snake without being able to tie it into EiH. And besides that, how many Smash 4 newcomers would have become one-time PCs? We haven't seen how any Smash 4 newcomers would have been handled in a more typical, ground-up game yet despite those characters having been added many years ago.
When it comes to Smash 4's newcomers, it's tough to say for the same reason you mentioned. Due to the nature of Everyone Is Here, it's tougher to judge how they'd be prioritized. However, with that being said, I do think
most would probably be prioritized enough to be kept.
Villager, Little Mac, Palutena, Shulk, Duck Hunt, and likely Robin I don't think I need to explain, but I will if prompted. Most seem to agree they'd be kept.
Pac-Man, Mega Man, Ryu, and Bayonetta would all also likely be kept, but their status as third party characters leaves at least some room for questioning. Either way I think they're among the most likely third parties to return, alongside Sonic. Cloud is often a hot topic when it comes to cuts, though I'm definitely more comfortable leaning toward him returning than many seem to be. Might just be me though.
Greninja and Rosalina & Luma I also think are surefire returnees, but for one reason or another many folks disagree. Greninja has, in my mind, achieved close to Lucario's status as far as popularity and notability within the Pokémon franchise, so I think Greninja's in it for the long haul. Him being a well-designed, relatively-simple character also helps, I imagine.
With regards to Rosalina & Luma...I'm genuinely confused as to why they think she'd be cut. She has an incredibly unique style, being Smash's only puppet fighter, and is a major fan favorite within her franchise. Her being a Mario character that wasn't made in the 80s is also a nice plus. Pretty much the only way I see her being cut is if the Mario roster gets cut down to just Mario, Luigi, Peach, and Bowser, and that...really seems unlikely to me.
Bowser Jr. is...admittedly more of a toss up, based on what we know. His fate is, in my opinion, heavily dependent on just
how much the roster is downsized. Sakurai has gone on record stating that Jr. almost didn't make the cut in Smash 4, and was completed fairly late into development compared to the rest of the roster. Ergo, we have proof of him having been low priority, as well as objectively lower priority than Rosalina & Luma. On the other hand, however...there's no way I can imagine a reality where Bowser Jr. is the Mario character with the lowest priority. Not when Doctor Mario and Daisy are clones, and Piranha Plant was similarly low priority but on a much larger roster. He's a toss up.
Probably my most controversial opinion so far, but I do not think Wii Fit Trainer is at risk of being cut. Could I be wrong? Of course. However, I do think she has precedence on her side. For first party franchises at least, cuts tend to not eliminate an entire playable franchise wholesale. The only instance of this happening for a first party franchise was Ice Climber in Smash 4...and it's very well documented that Sakurai moved hell and earth to try, but ultimately fail, to get them to work on the 3DS. Wii Fit Trainer wouldn't have any such limitations, so I really don't think she'd be cut for that, and precedent has shown that she likely wouldn't be low priority due to being the sole representative of her series.
With the Mii Fighters, this is the
one case of a Smash 4 Ice Climbers I can think of as far as "has a chance to be cut due to limitations." However, said limitation is, for now, unknowable...it's whether or not the next Nintendo system will include a Mii Maker at all. It's a fairly well known dilemma at this point that ever since the Switch's release, Nintendo has been further and further distancing themselves from the Miis, likely as a byproduct of wanting to distance themselves from the Wii branding after the Wii U was a catastrophic failure of a console. If they maintain or reverse course, then I think the Miis are for sure coming back (though for time limitations, perhaps only as a single moveset instead of three, but again, that's dependent on how
many cuts there are). If they double down on ousting them to the point of not including a Mii Maker on the console though...then I'd be worried for them.
Dark Pit and Lucina are likely going to be deemed low priority due to their status as clones, which has precedence on its side. It's possible they could be kept, like Falco or Ganondorf going into Brawl, but at the same time, if any echo fighters were to be given higher priority like that, I'd imagine it would be Chrom and Dark Samus due to their ballot status. We'll have to see.
Corrin...oh, Corrin. Corrin is the one Smash 4 newcomer who's unique that I firmly lean toward getting cut over being kept. Sakurai's sadly been on record about his view that Fire Emblem may have too many characters, and of them, Corrin is easily the least popular (very easy to see when looking at Choose Your Legends, which, yes, should be considered, as IS has clearly used this for non-Heroes purposes at least twice this year alone), comes from a very polarizing set of games (making them controversial even among Fire Emblem fans), and to top it all off, seems to have the least people playing as them, going by tournament results and online multiplayer censuses (the latter of which is data that Nintendo will absolutely be looking at, especially since theirs is even more precise than the online estimates). All of this adds up to a character who may very well get deemed as low priority.
So basically, the main ones I'd be worried about are Dark Pit, Lucina, Bowser Jr., Corrin, maybe Cloud, and depending on whether or not the next console has a Mii Maker, maybe the Mii Fighters.