Even if it’s true that a designated retro slot isn’t a thing, that doesn’t mean that we can’t still get retro characters. I feel like too many people falsely correlate the fact that something doesn’t have to happen with the idea that it means it can’t happen. The same goes for people that assume that just because something hasn’t happened yet, it means that thing will never happen. I feel like a lot of people focus too narrowly on trying to find patterns and dismiss anything that doesn’t fit into those patterns.
As for Takamaru himself, I’ve never played his games but I like the idea of a playable samurai in Smash. I wonder if he’ll get a reworked design like Pit did in Brawl. Takamaru’s classic design is just a little boring visually for my personal taste.
That was why almost all the newcomers came out of left field last time. People just focused too much on patterns.
Joker: Due to Persona being an almost Playstation-exclusive series no one expected him to get in. If Atlus was to get content they instead expected to see Shin Megami Tensei getting a rep instead because that series actually had mainline entries on Nintendo systems.
Byleth: As the previous four DLC characters were all third parties many people expected it to be the same for the final character. The only thing pointing in Byleth's favor was the absence of a Spirit Event for Fire Emblem: Three Houses while other games released around the same time like Astral Chain, Pokémon: Sword and Shield, and Daemon x Machina got one.
Min Min: ARMS was seen as a base game or bust deal, so many people were shocked to learn that it would indeed be getting a fighter. This caused a mad dash of speculation, especially considering that the obvious frontrunner in Spring Man was an Assist Trophy. Min Min came out of left field since she was a Spirit in the base game, and many people at the time believed that Spirits can't be promoted to fighters, despite both Mewtwo and Lucas getting to be DLC in Smash 4 despite both having trophies in the base game.
Sephiroth: Due to Final Fantasy's lack of content in the base game and general issues with Square Enix no one expected the series to get a second rep. Instead at the time a lot of people were pointing to Jonesy from Fortnite to be the next character. Not even a post claiming that the song choice in the trailer would be a hint to the fighter pointed people in the direction of Sephiroth, making him a massive shock to the community.
Pyra and Mythra: After ARMS got a rep, there was heavy speculation on Xenoblade Chronicles 2 getting a character, as Sakurai wanted to rep both in the base game but couldn't due to time constraints. However, virtually expected it to be Rex who got in. Pyra and Mythra came out of left field for both this reason and the fact that they were a transformation fighter, as many people thought that the concept was dead following Smash 4's decision to separate Charizard, Zero Suit Samus, and Sheik and that the concept only came back due to them having to bring the Pokémon Trainer back for the sake of Everyone is Here.
Kazuya: Tekken was a popular choice in speculation for a second Namco Bandai character, and many thought that it would get one because at that point it was the only remaining veteran third party company that hadn't gotten a newcomer yet, whether they be in the base game or DLC. However, most people thought that it would be Heihachi, who was released as a Mii Fighter costume during Min Min's wave. Sakurai also planned for Heihachi to join the roster in Smash 4, but scrapped him due to moveset problems, which led to many people doubting that Tekken could get a character at all.
Sora: Sora was the biggest shock of the whole game. He came out of left field for many reasons, one of them was Disney's involvement. People debated over whether or not they would have to include classic Disney characters as Spirits or Mii Fighter costumes. Many people also didn't think that such a big character would end off the pass due to Smash's tendencies to save the promotional picks for last, and they instead turned to the last DLC character being a promotional pick like Corrin and Byleth, although they instead pointed to the protagonist of Shin Megami Tensei 5 to be the final character instead because there was no new Fire Emblem game in sight.
I did not list Hero, Banjo and Kazooie, Terry, and Steve due to them being either leaked or decently expected by the community.