Waluigi. Should be fairly self explanatory (why would I pick a generic Toad over a fan favorite?)
Geno. Paper Mario was inspired by Super Mario RPG (even being called Super Mario RPG 2) so I feel Geno is not only more important but more unique. Also they'd likely be basing Paper Mario heavily off of Sticker Star and Colour Splash, which I'd rather just not have at all.
I don't want to choose here, but..
K. Rool. He's overall more important to the Donkey Kong Country series, being the main protagonist and all.. even if he hasn't appeared for a while. I'd really like to see them both be added eventually.
Ridley. Sylux is very creepy and would be a good rival for Samus, but he ain't Ridley. I feel Ridley's status as the main antagonist of the Metroid series along with one of maybe two consistently reoccurring characters (if you don't count Mother Brain / Kraid) and having a more recent appearance (both in timeline and in recent games) makes him much more valuable than Sylux.
Impa. I like Tingle and all, but Impa makes much more sense to me personally. She may not have her own games like Tingle does, but she's still very important to the Zelda series regardless and going back way farther than Tingle, even getting more recent appearances as well. I think even having a frail old lady being an incredibly ninja would be entertaining if they decided to keep her BotW appearance.
Decidueye. I like him more, and Rowlet was my starter of choice.
Bandanna Dee. Marx would be cool, but I think people really overstate his actual importance. He's had one or two actual main appearances that aren't very recent outside of the Kirby DLC which likely
started development too late to be considered. Bandanna Dee would also round out the Kirby cast to the 4 playable characters from Return to Dreamland, so it works for me.
Neither. I like both of their roles in the game already, and I don't think either really fit as fighters at all. If I had to choose, I'd pick Tom Nook because he's been in since the beginning and is arguably more iconic, however, I'd much rather see the Mayor. I'm fine with Animal Crossing just staying as the Villager at all, honestly. It doesn't really need more fighters, just give me more stages/items/music and I'm happy.
Spring Man with a
Ribbon Girl alternate costume. They're too similar to really be different in Smash characters IMO. I'd be fine with Ribbon Girl making an appearance as a clone, but I'd rather have her as a costume than not have her at all.
Hades. Pit. Hades! Pit!
Hades... Hades is just a much more interesting character overall. I have no other reason.
Wolf. Krystal gives me the heebie jeebies, and I just find Wolf to be a much more fun and interesting character both in Star Fox and Smash.
Samurai Goroh. He's more interesting than Black Shadow and has been around since the original F-Zero. Not much else to say.
Spyro. I think both have a near 0% chance of being in the base roster (there is absolutely no way they weren't too late to even be considered, let alone would have been considered at all), but I just like Spyro more.
Rayman. Is this a trick question?
Simon Belmont. Bomberman is cute and I like both of the characters, but Belmont sounds like they'd be more fun to me. A whip character is something I feel we need. Can't we have both? I want both.