Just saying, I'll be fine with any outcome except for Edelgard, honestly. The landscape of Smash speculation is already a wasteland, so if Edelgard got in as DLC, it'd be that much worse. It would be even MORE salt-inducing than Corrin, I can guarantee you that, and this is coming from someone who is excited about Edelgard as a character in her own series. I have nothing against her, but her inclusion would be... hoooo.
1) She's another FE rep, which quite a few people are, in general, feeling adverse to until the next game (wherein we'll more than likely see cuts, including trimming down the fat of Marth cloneage).
2) She would be a 'shill pick'. Three Houses is not even out yet. Bringing in Edelgard right before or after the release of Three Houses without much chance for anyone to get attached to the character, without much chance for a public opinion to begin with, is right along the same line of thinking as Corrin. She would be just as much of an advertisement pick as he was.
3) Both of the above when the outcry was large against Corrin. Now, I'm not saying everyone hated Corrin or that the consensus remains the same outside of anecdotal experience with internet opinions, but... I rarely, if ever, saw praise for Corrin making it as DLC. It was mostly a lot of hatred. That'd induce salt due to Nintendo repeating the same 'mistake' again.
4) Rex was denied from Smash, FP included, because he was too 'recent'. The same applied to Springman, as well. Edelgard is even MORE recent. The massive middle finger to ARMS and Xenoblade Chronicles 2 fans would be insane.
5) The Fire Emblem fans who don't mind more representation or love Edelgard (as I'm sure she'd have her stans) may continue to build upon those flames. (I like Fire Emblem and I'd like more representation that's not just sword fighters, given the huge spectrum of classes, but it's just not time for that now.)
I think she's unlikely, but I wouldn't count her out. If she does happen, uh... whoooooo. I'm gonna be staying far away from social media and social venues of any kind online for a WHILE. It'll be a war zone.