Sakurai and his team originally had different plans for a Xenoblade Chronicles 2 DLC character. He tried to include Rex as a playable character alongside Pyra, at first thinking of an Ice Climbers-like system where the player would control both characters at the same time, but this ended up presenting programming problems including one involving the game's memory capacity due to the size of the characters. He then had an idea of making Rex a background character whenever Pyra fights, in a similar manner to the Pokémon Trainer. He eventually felt this second method would also be taxing to the game and that fans would not be fond of it anyway so it was scrapped. He then came up with the idea to have Pyra be a transforming character (similar to how Zelda and Shiek were before Super Smash Bros. for Nintendo 3DS and Wii U) with her turning into Mythra and additionally have Rex appear only in taunts, Final Smashes, and other extraneous details involving the characters.

By March 2019, with just 13.81 million units sold the that time, Super Smash Bros. Ultimate had usurped Super Smash Bros. Brawl (if one does not count the different versions of Street Fighter II and Super Smash Bros. for Nintendo 3DS and Wii U which Ultimate also surpassed) as the best-selling fighting game of all-time.

During the beginning part of Sephiroth's Supernova Final Smash where everyone turns to the Z-Axis to view Safer Sephiroth, the normally "flat" Mr. Game & Watch can be seen as a 3D model if he is caught in the attack.

According to Masahiro Sakurai, the reason Min Min was chosen as the playable Arms representative out of the game's cast is because Arms producer Kosuke Yabuki personally requested Min Min; Ninjara was among his top choices to appear in the game as well. Yabuki at one point considered asking them to add the entire cast of Arms' lead protagonists as alternate costumes or playstyles instead of just one character.

Although Terry Bogard from Fatal Fury had made it into the game as a downloadable character, Masahiro Sakurai had initially planned to add Mai Shiranui alongside Terry as one of the cameos in the King of Fighters Stadium stage. In the Japanese Nintendo Direct presentation of Terry Bogard, he explained further that he worried that CERO (Japan's video game rating board) would raise the rating from A (for all Ages) to a higher one which was why he reconsidered having her in the game, joking that Super Smash Bros. was only for "good boys and girls".

When Piranha Plant was revealed as the first DLC character, many fans were bewildered, as a generic enemy has never been a playable character in the Super Smash Bros. series before. Despite this, in an interview with Game Informer, Masahiro Sakurai has stated that he never intended to surprise people with the Piranha Plant, and that it was a character everybody would know of.

The plot of the Adventure Mode: World of Light, in which Kirby is the sole survivor of a cataclysmic invasion and must rescue the other fighters, was actually series director Masahiro Sakurai's original vision for the plot of Super Smash Bros. Brawl's Adventure Mode: The Subspace Emissary.

This early storyline was mentioned during a 2008 Iwata Asks interview:
"I had envisioned a more serious tone for the story. Something with some misfortune like a single character escaping total annihilation of his squadron and then fighting back while rounding up his allies."

It should be noted however that Spoiler:this early storyline would be implemented somewhat closer to the end of The Subspace Emissary anyway when Tabuu decimates every fighter turning them back into trophies, while Kirby, Luigi and Ness are revived in separate locations thanks to the three Dedede Brooches.

Jun Senoue, famous for being a lead music composer for Sonic Adventure 1 and 2, said he was insipired to write and arrange Super Smash Bros. Ultimate's "Mega Man 4 Medley" in the style of a Sonic Series composition after he came across a Sonic and Mega Man crossover comic that sparked his imagination. He chose to center his arrangement on the 4th Mega Man game because of fond memories that he had of playing the game at university.

Abby Trott, the vocalist of the English version of Lifelight, stated in a interview that she actually cried upon hearing that she would be involved in "Super Smash Bros. Ultimate." Trott, herself, was a Nintendo fan and hearing about this made her feel "very special."

“I was lucky enough to have the opportunity to audition through Cup of Tea Productions, and at the time I had no idea what the audition was for. For the first round, I submitted my singing demo. The second round involved singing a requested song (not ‘Lifelight’). I ended up being cast, and CRIED when I found out what the project was. As a life-long fan of Nintendo, being a part of Smash Bros. Ultimate is really special to me. I love ‘Lifelight’ so much.”

Many of the unique titles for each fighter in Classic Mode contain Easter eggs and references. A few examples are:
• Mario's title "Let's-a-Go" is a reference to one of his phrases.
• Link's title "A Quest to Save Darkness" references the plot of most Zelda games where he's on a mission to seal Ganon away.
• Dark Samus' title "The Great Poison Given Form" references the essence she was created from in the Metroid Prime series.
• Kirby's title "Gourmet Clash" references the Gourmet Race minigame from Kirby Super Star.
• Pikachu's title "I Choose You!" is based off Ash Ketchum's signature catchphrase from the Pokemon Anime, as well as the name of the Anime's first episode.
• Luigi's title "Luigi's Nightmare" is likely a reference to his cowardly personality.
• Ness' title "Home to Onett" is a reference to the Homesick status featured in Earthbound.
• Jigglypuff's title "All Original, All 64" is a reference to the fact that Jigglypuff is a member of the Perfect Attendance Crew for being in every Super Smash Bros. game since the original for the Nintendo 64, and the opponents it faces are against other members of the Perfect Attendance Crew.

According to Masahiro Sakurai, in an interview with Famitsu, the reason why Kirby (being a creation of Sakurai's) was the only one that survived Galeem's attack was because he was the only one who could plausibly escape it using his Warp Star. This also made the character accessible to starters or newcomers to the game, as a result of his speed and attacks.

Charizard, Incineroar, the Inklings, and Sonic the Hedgehog are the only characters that take continuous damage when in water. This is in reference to their respective series, as Sonic cannot swim, Inklings are instantly killed when they come in contact with water, Incineroar and Charizard are fire-type Pokémon which are weak against Water-based attacks.

If one hacks the camera in the recreated Great Bay stage from Melee and points it to the far left, it's possible to see geographically accurate landmarks originally seen in the world of Termina from The Legend of Zelda: Majora's Mask that aren't seen during normal gameplay.

This includes a series of ledges leading to an alcove with a Heart Container inside of it, Fisherman's Island, and the large gated entrance to the Pirate's Fortress, complete with a new design featuring two crossed swords.

These do not appear at all in Melee.

A voice line of Snake saying "There!" can be found in the files for Super Smash Bros. Brawl. While it remains unused in this game, it would later be used for when Snake plants a C4 explosive in Super Smash Bros. Ultimate.

Taking the camera inside of Dungeon Man on the Magicant stage will allow you to see Brickroad, who was the operator of the man-dungeon hybrid in EarthBound.

Files for Dragonite from the Pokémon series suggest it may have been planned to be summoned from the Poké Ball. While it does not appear as an assist character in the final game, it does appear as a background character in the Kalos Pokémon League stage. Two models for the character exist, with one more in line of those summoned by the player, featuring higher resolution textures.

Unlike other boss characters, Spoiler:Giga Bowser is classified as a fighter in the game's files, even having placeholder slots for alternate costumes.

These are likely leftovers from the previous game, Spoiler:where Giga Bowser appeared as a playable transformation.

Ridley's orange palette swap is in reference to the villain's appearance on the box art of Super Metroid.

Yoshi's new Final Smash, where he summons an army of multi-colored Yoshis, is a reference to his part in the opening of Super Smash Bros. Melee.

When Pit gets Star KO'd, he'll shout out that he never learned how to read. This is likely a reference to a scene from Kid Icarus: Uprising Spoiler:when Palutena cuts off his power of flight upon defeating Pyrrhon.