Unfortunately the common theme has been that either the character is relevant (as in had a recent release), or it's a retro character from way back in the day, Pit was classified as the latter when he was included in Brawl. I'd really love to see someone who's caught in between to get in though, like K. Rool, Isaac, or even Starfy like you brought up.
Although a lot of us like to complain about the tribalism of Smash character fan bases, speculation for this game has been based a lot more on logos and ethos rather than the pathos of pre-Brawl and pre-Smash 4 and the move to accept recency as the primary criteria for selecting characters has played the biggest role in that, especially since we got Bayonetta as the Ballot character who like K. Rool, Isaac, Banjo, and others had a recent Wii U appearance. Since it's clear that what we say now won't affect the base roster and that Sakurai rejected one of the most popular Nintendo request in Ridley, the shift in the character scene switched from the pathos-influenced "Let's get our favorite characters in!" to the logos and ethos-influenced "Based on who has appeared recently or who hasn't appeared since the Famicom/NES, who is the most likely?" After this game, especially if there's no ballot character outside the Inklings - who would have gotten in regardless - I don't think character advocacy will be much of a thing anymore and this will result in a significant decline in how many people decides to talk and speculate about prospective characters. Think about it, if all we can get for newcomers in future installments are characters from recent games, why bother supporting your long since dead favorite character? I know I wouldn't be here if there weren't several newcomers I wanted that I thought had a shot.
That tangent aside, I would say that King K. Rool and possibly Geno and Banjo are in an unique position among "characters without a future" if only because they are heavily requested, the former two were noted as popular requests and the latter is not only obviously one of the most wanted, but said inclusion would also be consequential towards the video game industry if only for what it means for Nintendo and Microsoft's relationship. Everyone else that isn't a veteran has no shot.
Characters from inactive franchise (unless they're basically from the NES era) are kinda screwed based off Sakurai's multiple statements. Inactive characters from active franchises on the other hand are somewhat better off and can still be considered and wanted by Sakurai. If that wasn't the case, Geno wouldn't have even been considered, let alone wanted by Sakurai.
Actually I'm curious, is Sakurai statement noting that Geno was a popular request a hint that he did well in the Ballot? We know Sakurai wanted him for Brawl but I wonder if like supposedly King K. Rool, Geno's Mii Costume was in response to the Ballot. It does, after all, have unique assets to it.
Maybe backlash was a strong word, and like I clarified it wasn't really a response to that either way, it was just the best word I could find to describe the fan reaction given that English isn't my native language.
To say that we're guaranteed two FE newcomers just because that's what happened in Smash 4 is a pretty hasty conclusion to reach though, a new game always shakes things up pretty drastically and brings new norms with it. People who used Brawl logic to predict the Smash 4 roster were horribly wrong, and there's a decent chance that using Smash 4 logic to predict the Smash 5 roster will be just as wrong, god knows what part of the roster we'll be complaining about in a year's time.
I see. I would have used something like "many fans were disappointed" or "upset" that Sakurai added Corrin.
Honestly, I'm willing to bet real money on us getting two Fire Emblem newcomers (and for that matter, two Pokemon newcomer). Even if we assume that Sakurai will use different logic to include characters, the concept of Sakurai modesty was proven a myth with last game with how much representation Kid Icarus: Uprising and Fire Emblem got and also how Sakurai decided contents for Kirby. That doesn't go away just because it's a new game even if say, we see more emphasis on popular requests from the Ballot than recent appearances. It's obvious given how Sakurai talked about Fire Emblem that he is affectionate for the series and thus would see more out of the latest Fire Emblem character than say, most other franchises. If last game can get us four Fire Emblem characters, two is not unfeasible to imagine.