Do you feel like Ratchet & Clank could potentially work for smash 3rd parties? Their games aren’t overly violent, they have a cartoony pile of weapons for moveset potential, and it would be fun to just hit things with an omniwrench.
They could, but are Ratchet and Clank that relevant? I suppose they had a game in 2016, but they don't seem that iconic for me (and if we're getting a third party rep, especially a third party rep from Sony who is probably Nintendo's main competitor at this point, they have to be very iconic). For me the Sony characters that would make more sense would be either the Prince of All Cosmos, Sackboy, or someone from Silent Hill (Pyramidhead? He's not cartoony at all, but man oh man would he be a terrifying addition. And okay, a Silent Hill character is more a personal want than one that's actually likely).
My feelings with third party characters is that they need to either have an important role in Nintendo history (Simon Belmont, for example) or have an "icon" status in video game history (Like Sonic, who when he was added in Brawl didn't have too many Nintendo games out, I think). In adding any character we know the thoughts that go into it is about Relevance (this is where we look at a game's importance in the current revenue stream or- like with Ice Climbers, etc.- in Video Game/Nintendo history), Popularity (How much this character is wanted by fans, and how much hype this character would generate for the game), and Moveset (Would this character be unique and would they add an interesting mechanic in the game). With first party series (like Fire Emblem or Dr. Mario) it's okay if one or two of these categories are lacking, however with third party representation all of these categories have to be on point.
So, with the reduced roster of newcomers for the base game I don't think we're going to be getting any more third parties (Though I didn't think we'd get any third parties, and Simon Belmont lashed out). My personal theory with DLC is that we're going to have a lot of upgraded Assist Trophies/Pokeballs, opening up the chances for characters like Bomberman (a third party who- for me- certainly deserves a spot on the roster), Eevee (who is very important in Pokemon history and a great representative for an important Pokemon mechanic. Also who is one of my favorites), and of course Ashley (Who was popular before being deconfirmed, and has been steadily growing in popularity since).