I'm not generally opposed to a Fire Emblem newcomer, but I'm not generally supportive of it either, I treat it like any other franchise, there are characters from it that I want, and there are characters from it that I don't want. Black Knight would be the most hype newcomer they could include for me bar none, and I think Lyn and Micaiah would also be really cool, but I did not care for Robin or Corrin, and if we get Byleth I wouldn't really care for him either. I don't really care about franchise representation much, I support characters based on their individual qualities, so when talking about a series potentially getting a newcomer, I don't like to look at it as if it can only be black and white.
Obviously the characters that I'd like from Fire Emblem stand no chance though since the series' presence in Smash is driven almost entirely by how it can promote the newest title, which is where my problem with how it's handled in Smash lies. Fire Emblem still has a lot of iconic characters that aren't in Smash who could really add a lot to the game imo, but they're never going to get the chance. I liked Chrom's inclusion because he sort of broke that, he's only an echo and Awakening is still new-ish, but the reason for his inclusion was obviously due to his popularity above anything else.
I also feel that characters included primarily for promotional purposes are like a coin flip, usually they're planned before players even get the chance to try their game, so there's no way of knowing whether they'll be a hit or not. Sometimes they are, sometimes they're not, and sometimes we never get the chance to form an opinion either way because their games weren't even localized. That's not to say we should never get these kind of characters, I think they have their place, but I don't like when they're the only options, as is the case with Fire Emblem.
If you asked me right now whether I'd take a Fire Emblem newcomer as part of the pass, I wouldn't really know, because it's going to be a Three Houses character, and I obviously haven't had the chance to play that yet. I think my opinion on Byleth is unlikely to change since he's of a similar archetype to what we already have, even if they can make him play differently I'd like someone that's more visually striking compared to the other FE characters. Edelgard might grow on me since I'm planning to go with the Black Eagles, but it's one of those situations where it feels like a coin toss, right now I really can't say whether I think she'd be a cool character to have in Smash.