There aren't really too many newcomers who would genuinely upset me other than a few obvious ones like Rabbids or Sans. There are a few reasonably popular picks that would likely kill my hype, however:
Waluigi - This is mostly out of pure and utter distaste for the character itself. From what I've seen, most everyone seems to either love the guy to death or hate his guts, and I stand firmly in the latter camp. I always found his dumb "WAH" shtick to be more obnoxious than funny, overexposure due to memes hasn't exactly helped my impression of him, and the fact that his entire existence is derivative of two different characters, Wario and Luigi, whose existences are themselves derivative of Mario, not to mention that he's basically been relegated to the role of Wario's Tennis Partner since his inception, just makes me feel like he doesn't deserve to stand in Smash, quite frankly. Give me a call when this man does something interesting, like, say, appear in a main-series title. That, and of course, there's a certain other Mario character I'd rather see get in before any meme-spawning purple genie.
Captain Toad - I'm sorry, and I know saying this is a cardinal sin for any character these days, I just cannot see him fighting. I can at least see Waluigi fighting, as much as it sickens me to do so, but Captain Toad has practically no usable moves in his own game beyond picking up and swinging a pickaxe over his head (woohoo) and throwing turnips, which is something another Mario fighter already does. I'm sorry, and I usually have a pretty high tolerance for creative liberties when it comes to moveset potential, but when we have entire movesets being pulled out of *****, that's where I draw the line, and I just can't see any other direction possibly being taken with Captain Toad. And then there's the simple fact that he can't jump, which is an even bigger issue in and of itself. If Captain Toad gets in, I believe it will bode very badly for future newcomers.
While I'm at it, I might as well cover Toad as well. He's a member of a species, not a character. At least Captain Toad is a distinct character, even if he did start as just a Toad with a headlamp. That, and he's already Peach's Neutral-B, and that's not gonna change anytime soon.
Daisy - GENO OR BUST! In all seriousness, she's annoying, albeit not anywhere near as annoying as Waluigi, she's not liable to have a unique and/or interesting moveset, and, to reiterate, I'd rather have Geno and/or Paper Mario as a new Mario rep. Next.
Mimikyu - I know what you're thinking, and no, this isn't just because I have stock in Decidueye. If it were, I'd have Lycanroc here, too. This is more of a Waluigi-type deal where I just cannot stand Mimikyu and, frankly, find it incredibly overrated. The Pokemon Company forced - err, sorry, the word's marketed now, isn't it? - the hell out of this thing unto the public before SM dropped, and wouldn't you know it, the Internet took the bait hard and instantly bought into the "d'awww it just wants to be loved and it's so kyoot!! >w<" hype. Well, not me. I find it just as bad as any other common Pikaclone, if not worse, and choose not to give it a pass just because it wants attention. I also think it would work much better as a Pokeball summon due to its downright puny stature and unconventional build, so I hope Sakurai made the right decision here.
Ridley - Yep, sticking to my guns on this one. I'm in the apparent minority who thinks that Metroid does not "deserve" any more fighters than it currently has, and even if it did, I'd rather it be Sylux, who's confirmed to be making a comeback as of Prime 4 and has a much more realizable build than Ridley, even if he's not as """iconic.""" Ridley would be... interesting, to say the least, but likely not in a good way. Yes, I am implying that he's too big, fight me fam.
Anyone New from Animal Crossing - I don't care how "big" they are, non-combative franchises should not have more than one spot in Smash, especially not for the sake of "muh representation". Not much more to say about this one.
A BOTW Champion - "But muh relevance doe!" Yeah, and then they'll lose all relevance and be decidedly out of the public eye by the time the next Smash rolls around. Adding one of these guys would be an incredibly short-sighted and, in my opinion, poor decision. At least other Zelda one-offs like Midna and Ghirahim have staying power to back them up.
Sora - No.