Alright, here's the main takeaway: Edelgard is the new Lyn and I think the one 4chan post was just koala-bearing off people's basic expectations of DLC because it's "safe." People have basically attached themselves to the character because she appears to be the most unique between her, Dimitri, and Claude. The trailer shows she can use axes and I think it's been suggested that she can do magic too. A magic using axe user does sound badass but she's not the main character. She's a secondary character. The main character is Byleth. The story is told from his perspective. It's unclear if he's a customizable avatar or not but that's irrelevant. Smash has never added secondary characters first for Fire Emblem. Lucina came with Robin and Chrom came with this game (in more ways than one, ZING!). Lucina and Chrom are both echoes. That should be telling enough. Marth, Ike, Roy, Robin, and Corrin are all the lead characters of their respective games. Lyn has been an assist trophy since Brawl and Edelgard's role in Three Houses is almost identical in premise to the one that Lyn had in Fire Emblem: the Blazing Blade. This all being said, if Three Houses gets a playable character, it's going to be Byleth based on 17 years of character additions in Smash. It's literally Lyn, Hector, and Eliwood all over again.
Beyond this, does Fire Emblem really NEED more content? Is there really that much they can do with the series in Smash at this given time? It has 4 totally unique characters, 2 echoes (1 is all new), a semi clone, 3 stages, 3 assist trophies (2 are brand new), an item (brand new), 40 spirits, and 37 music tracks (with 10 brand new remixes). That's incredibly generous by all accounts especially when it's not even one of Nintendo's S tier series. Keep in mind that the DLC packs are a character, a stage, and more music. When Corrin came, it was just the character and like two music tracks. There is absolutely no reason to show this degree of favortism towards Fire Emblem. It doesn't need advertising. Sakurai practically put a gun in his mouth when they asked him to put in Corrin and now they're supposed to do yet another? Chrom was copy and paste with some adjustments, this is an entirely new character from the ground up. Corrin was also meant to not just advertise one game but technically THREE so that makes a lot more sense than one game on what's rapidly becoming what may wind up being Nintendo's most successful platform of all time.
This all said, my arguments against Sylux are practically the same. Metroid now has 3 unique characters (1 new to this game), 1 echo (new to this game), 4 stages, 2 assist trophies, an item, and 25 music tracks with 7 being new to Ultimate, It's also a series that's plenty well represented given its size and Metroid Prime 3 doesn't really need advertising given the Switch is a runaway success. This isn't to say that Edelgard and Sylux can't happen, I'm just saying people are banking on them way too hard despite the fact that both Fire Emblem and Metroid have had an avalanche on new content in this game. On top of this, Roy and Corrin are literally the only two "shill" characters in the entire history of the series. 5 of the 7 Smash 4 DLC characters were pure fan demand with Ryu being the most iconic fighting game crossover character ever, bar none. Almost 20 years of Smash and now people are expecting them to do two shill characters within months of each other, despite the fact that we just got Joker, a character whose main game is currently only available for the PS3 and PS4? In the end, it's not impossible for either but I think people need to take a step back and realize that both would be huge anomalies with everything that I've mentioned here.