Hmmm, interesting question, because I realise that my answer changes depending on the situation.
For example, I normally would love for Tingle to be playable. He's the next most important Zelda series representative, is the lead star of his own games, has plenty of move-set potential, and is a character I simply like. Normally I would love to see him included, but then I see how the Zelda series already has 6 representatives and it really dampers my interest in including him; it feels... unfair to the rest of the less represented series for it to be so heavily skewed. I saw Super Smash Bros as a celebration of Nintendo; as a showcasing of ALL it's IPs and history, sharing the limelight on a near even footing. Obviously that latter point is clearly not the case with how Sakurai has handled the series, and has always bothered me.
Secondly, I don't like removing characters from a non-story focused fighting roster; even when I disliked their inclusion in the first place. Unless they are a guest character (which all the 3rd parties should've been), then the characters should be kept on from each installment, given there is no story to justify their exclusion in the SSB series. I hated the addition of Rosalina, but now that she is in SSB, I would be disappointed if she was removed. I think this mindset is why I've always placed such importance and care onto the addition of characters; because once they are their, I feel they should stay.
So onto the question itself, who do I not want in the game. Well I don't want any more representatives from the already heavily represented Mario, Zelda, Pokémon and Fire Emblem series. I would have honestly stopped at 4 reps max for any series myself. I would love to see Tingle playable, but unless they have him representing his own series akin to Yoshi (though I question that decision, given how connected Yoshi is to Mario series, unlike with the Donkey Kong Country or WarioWare series), I wouldn't want him when there are so many series that could do with playable reps.
As another broad stroke; 3rd party additions. It's quite clear that they are not guest characters at this point, they came and they stayed. It gets in the way of the focus on Nintendo's own IPs, from the big ones and particularly obscurer ones. There are certain 3rd party additions that I'd enjoy seeing (Bomberman, Rayman, Shantae), but I wouldn't want them in SSB due to their 3rd party nature. If these 3rd party additions were guest characters and only popped up in one entry, then I'd be fine with them, but they don't serve the role of guests.
I struggle to think of any specific characters who I would dislike the inclusion of, they tend to fit into broad categories... Geno perhaps, but I feel that fits into that previously mentioned 3rd party aspect. I'll have to think on this one.
First point - it's hilarious that you say I don't know how it works when I am confident that I know far more than you considering you don;t know when something is OBJECTIVE or not. Last I checked it took development time to create both Zelda and Sheik, alst I checked every link takes development time as well. I ahve ntohing against having a few 'echo fighters' int he game as theya re obviously easy to develop and a quick way of putting in new characters like Daisy, but how about isntead of making several versions of the same characters you make new ones? How about instead of 3 Links, we get King K Rool? Or what King K Rool take more development time than 2 of the 3 Links would? And even if it would, how much more would it take and would that effort not justify having a new, completely different character?
On to your second point:
Wrong. There are OBJECTIVE qualities that would make a character more deserving of being in Smash than others. You are simply wrong to say that.
Shantae for isntance is far more deserving of being in Smash than WFT.. who isnt even really a character to begin with. By your logic, no character is more deserving of being in Smash over another. So then by that logic, why is Mario in the game? Why is Link in the game? Why isnt Little Mac's coach a fighter in smash instead of him? It's because there ARE objective qualities that characters have that make them more deserving of being in the roster than others. If that wasnt the case then the entire roster would just be nameless background NPCs from random games. Do you want me to talk about how big of an icon Mario is in the gaming industry, how many games he's been in, how he's the face of Nintendo, how he has so much potential for a unique moveset and would easily fit into smash, etc........... but none of that matters because it's all subjective, right? He made it into smash only because Sakurai likes him, right? Nothing else came into play, right?
Shantae is a female character (something there are not many of in smash compared to male characters), she's a female protagonist (something that there is literally only one of in smash), every single one of her games is on a Nintendo console and up until recently exclusively so. In fact she has been a Nintendo character for close to 2 decades so she has a history with the company as well. Her devs actively want her in the game, her aesthetic and the tone of her games perfectly fits in with Smash, all her games are well received, etc. She literally fits all the criteria that sakurai is looking for, she would fit tons of niches, she has so much potential for a unique moveset, etc. She would add a lot to the game regardless of if you personally like her or not, There is a difference between subjective opinion and objective facts. I personally don't care if King K Rool makes it in because I don't care about the DK franchise but I will admit that objectively he deserves to be in it because he would help fill a lot of niches in the roster, and not to mention he fits all the criteria of a character that Sakurai is looking for.
First off, I agree with your first point in that there is clearly a slot limit, but said limit is admittedly defined by development time which is harder to gauge. I also agree that it is ridiculous that we have multiple versions of the same character; the initial inclusions of them always irked me. With the theme being 'Everyone is Back', I have less of a problem with them returning, but I'm of the opinion that those additional Links and Mario's shouldn't have been added in the first place.
Onto your second point though, I do have a level of disagreement to that. There is certainly an argument for a character to deserve to be in SSB, and I admit to having been frustrated often with SSB inclusions that I feel didn't deserve their inclusion (at least, over others that had yet to be added), but I do spot a certain clash in our opinion.
I've added it in bold, but whilst I do like Shantae and would appreciate her as a 3rd party guest (I so wish they were actually treated as guests), I do not feel she has more reason to be in Smash over a Nintendo series representing character such as Wii Fit Trainer. Wii Fit Trainer, whilst an unusual choice, represents a popular multi-game series owned by Nintendo. In Super Smash Bros, a Nintendo All-Star game with the power to celebrate and give attention to it's many IPs and history, I feel Wii Fit Trainer's Wii Fit representing value gives her precedence over
any 3rd party addition.
Shantae has plenty of value to being added, but if the desire is for a female character (and in particular a female lead protagonist of their own game), then there are still plenty of Nintendo owned characters who can fill in that role.
Ultimately though, neither you or I are in charge of the game, and I question how much stock Sakurai puts into these factors. He did showcase a desire for unique move-set potential in SSB4, but when it came to series representation, he had no problem adding numerous 3rd parties or bolstering the Mario and Fire Emblem series with many more additions. He has also included clones in every SSB to some degree, with truly only the recent Echoes as being ones we could argue take a truly minor amount of time to add (though there are still unique animations and such to add, so it's not like it's no time). So when it comes to characters that have elements that make them worthy candidates and even deserve to be added, it really comes down to how much emphasis Sakurai puts into that.