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.

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.

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.

Mirroring a similar easter egg on the Temple stage, the Sheikah text on a platform in the Great Plateau Tower stage says "SMASHBROS" twice.

According to Sakurai's Famitsu column, he chose to build the game off of the previous entry's mechanics instead of building the game from scratch as choosing the later would have caused a large portion of the game's roster to be cut. This would be due to the fact that more time would have had to be put into rebuilding each individual character.
He also remarked this decision allowed for more focus to be put on other aspects of the game.

Super Smash Bros. Ultimate is the first game in the series that requires players to select a stage before selecting a character in multiplayer matches. In previous entries, as is common in other fighting games, it was the opposite. This is an attempt by Masahiro Sakurai to change up the strategy of the game (specifically stage-character match-ups).

Solid Snake's series logo was changed from the FOX emblem that was used in Super Smash Bros. Brawl, to the exclamation point seen when being caught in most Metal Gear Solid games.

This is likely because the FOX emblem (which first appeared in Metal Gear Solid 3: Snake Eater) is also commonly associated with series creator Hideo Kojima and his company Kojima Productions, a former subsidiary of Konami. Kojima left Konami in 2015 and the removal of the FOX emblem to represent the series is likely a biproduct of the split.

One of the two villains in World of Light, Galeem, may be based on the Seraphim, one of the rank angels in Judeo-Christian religions. Both Galeem and Seraphim have four pairs of wings and a halo, and both devour darkness with light, turning everyone (excluding the game's fighters) into spirits.

Sakurai and his team were already working on the Byleth DLC before Fire Emblem: Three Houses was even released. The game director had to work with an early developer's build of the game, which he said took quite a long time to finish. This early build was in fact so Top Secret Sakurai couldn't even been seen playing it and he was even forbidden from taking it to his house.

Ryo Horikawa (voice of Captain Falcon) revealed that when he was asked to record lines for the Min Min reveal trailer, he thought he was going to re-record alot of Captain Falcon’s lines in game. However, because Captain Falcon’s voice lines have been reused since Super Smash Bros. on the Nintendo 64, he was brought in to simply record ramen slurping sounds for the trailer.