When it comes to Fire Emblem in future Smash games, here's basically how I see it.
Marth, Ike, Robin, and Byleth stay, no questions asked. Marth is the first lord and franchise face, as well as a Smash mainstay. Ike is a heavy swordsman that offers a unique playstyle and is one of the single most popular characters in the franchise. Robin comes from Awakening which revitalized the series, and also uses magic, which Sakurai was on record as having said was important to showcase for Fire Emblem. Byleth is the main lord of by far the most popular game in the franchise.
Where it gets dicier is with the remaining four: Roy, Chrom, and Lucina's movesets are derivative of other characters, and despite this making them easier additions, there's also precedent that cuts go for these types of characters first. Lest we forget Young Link, Pichu, Roy (lol), Dr. Mario, Lucas, and Wolf, who were all cut at one point or another. Some of them could return, but it's far from a guarantee like the four above.
Corrin, meanwhile, is the one that I am absolutely positive will be dropped despite being a unique character. They're a highly controversial addition from an equally highly controversial set of games, and Corrin also lacks the merits of the four I feel are safe, while also not being an easier addition like Roy, Chrom, and Lucina. Corrin is one of the only characters in Smash where I genuinely wonder if the people who want them gone outweigh the number who want them back, as their addition to Smash is controversial even within the Fire Emblem fanbase. Either way I think they're the cut that would upset the least amount of people, and let's face it, there will be cuts next time.
One curious thing, though, is the status of Roy and Chrom. Both are popular, but especially when looking at the context of Smash. Roy's return was heavily wanted for Smash 4, while Chrom was a big fan request for 4 and his ballot results led to him getting into Ultimate. But here's the interesting part: Chrom still features in Robin's Final Smash and victory screen, meaning that he'll need to be modeled for the next game anyway if Robin returns. Because of his model already being there, he'd have enough of a foot in the door where I could feasibly see him returning, even if it means he'd be returning without Roy. Chrom is the only Echo who I could see return without the character they're echoing.