Yes I know, but he does it steadily and doesn't make many huge jumps for each game like most people think he does (at least not since Brawl).
Most people thought we'd get American third parties. They're mostly just collectibles so far.
Most people thought we'd get a second unique character for a third party series. Didn't happen.
Most people thought any Pokémon from older generations would be considered after Brawl. Didn't happen.
Most people thought new Kirby characters would be added after Brawl. Didn't happen, for some reason.
The Ender Dragon DLC boss rumor didn't even happen, so unless anything past the fighter pass content is announced, I'll take this theory with a grain of salt. We can't be certain which habits Sakurai will break next, if any. Besides bringing everyone back and how WOL is set up overall, the main big things to happen this time are second characters for third party series (as Echoes, even then) and a regular enemy as playable.
I know that Sakurai is intent on doing as much as possible in regards to supporting this game, but I'm not sure if the reduced staff will be capable of adding new bosses, even if Porky can be recycled from Brawl.
3rd Parties were unprecedented prior to Brawl. It happened.
DLC was unprecedented prior to Smash 4. It happened.
Many people thought Megaman was as good as dead due to his treatment at the time by Capcom (especially because of his awful depiction in Street Fighter X Tekken). He got in, and was as accurate as ever to his NES self.
Most people never thought Pac-Man could get in. He did.
Most people could never imagine a 3rd party company getting a trophy without a playable character. Rayman happened
Most people never expected 8 player Smash to actually happen. It happened.
Most people never expected Stage DLC. It happened.
Most people thought Dr. Mario coming back before Mewtwo was inconceivable. It happened.
Most people never thought 3rd party companies could get a 2nd character. Ryu happened.
Most people never thought Ryu or Cloud could get in. It happened.
Most people never thought a character that literally is the face of a 3rd party moving to Playstation could get in. Cloud happened.
Most people thought Roy and Mewtwo coming back were impossible after Robin, Lucina, and Greninja. They came back.
Most people thought Fire Emblem getting 4 new characters and Kid Icarus getting 2 was impossible. It happened.
Most people never expected every Veteran in the series to come back, especially not Snake, Young Link, or Pichu. Snake in particular because of all the hatred for Konami and the falling out with Kojima. It happened.
And while Pokemon hasn't gotten any newcomers from Older Pokemon Gens, getting Squirtle, Ivysaur, and Pichu back is a blessing enough for me, since Pokemon from Gens 1 and 2 already make up the vast majority of the Pokemon lineup. Also you could count the female Pokemon Trainer as a reference to FireRed and LeafGreen, since she hasn't appeared since then.
Most people never thought Ridley or K. Rool would ever be added. It happened.
Most people never thought 3rd party franchises could get a 2nd character. Richter and Ken happened.
Most people never thought a generic Piranha Plant could ever even be conceived as a Playable character, Piranha Plants have never even been playable in Mario Party or Kart or even Baseball (or any game at all, even Goombas were playable in some spin offs), let alone Smash Bros. And most people never thought it could make it in over the likes of Waluigi or Captain Toad as a Mario rep, or Isaac and Shadow in general.
For just as much things you say didn't happen, even more 'impossible' things did happen, mostly over the span of just two games.
I'm pretty sure bosses take a lot less time than playable characters, tbh, especially since Porky CAN be recycled from Brawl. To say that Sylux and Porky could be playable while saying that making them bosses would be too much effort is strange to me, to say the least.