Reasoning
From SSB64, the only one that would be at risk is Jigglypuff as it’s historically been low priority. The original eight seem to be important to the developers, Luigi is the player two character in gaming, Captain Falcon is an SSB fan favorite, and Mother is important to SSB (11 veterans). I cut Jigglypuff here, but I could see it being a late addition like in 64, Brawl, and For.
From Melee, I think most would agree with my decisions to keep Bowser, Princess Peach, Princess Zelda, Ganondorf, Marth, and Mr. Game & Watch as the first four are major characters in flagship Nintendo series, Marth is the closest to a Fire Emblem mascot, and Mr. Game & Watch is the classic surprise fighter I also made a controversial decision to keep Ice Climbers, who seem to be the most valued cut fighter outside of Pokemon Trainer from the developers’ perspective (18 veterans).
From Brawl, very few would argue against Meta Knight, King Dedede, Wario, Diddy Kong, Sonic, and Olimar given that the first four are major characters from franchises with fighters (Wario is interesting where he’s a Mario Kart/Party staple but also has his own series in the form of Wario Land and Wario Ware), Sonic is the poster child for third party fighters, and Pikmin is highly valued by Nintendo. Additionally, Pit is a big part to SSB, and he isn’t going anywhere. Personally, I think the Pokemon Trainer is important enough to prioritize here given their role in Pokemon and how they represent multiple aspects of the series. I also kept Ike and Lucario to supplement the Fire Emblem and Pokemon selection given their legacy within and outside of SSB (30 veterans).
From For, I pretty much with the leads of the most relevant Nintendo series (Villager and Shulk), the Mii Fighters for the sales of their costumes, and Mega Man, Pac-Man, and Ryu for supplemental third parties as Mega Man was requested, and Pac-Man and Ryu sem like a higher priority for the developers (38 vetrans).
From Ultimate, Inkling represents the most recent blockbuster, Ridley was a requested pick since the Brawl era, Isabelle is a major character from a blockbuster franchise, and Byleth, Aegis, and Min Min help keep things contemporary (45 veterans).
I think this covers most bases pretty well, but we do get some tough cuts like Rosalina, King K. Rool, Sheik (personally), Little Mac, and a plethora of third party fighters.