There is more to it than just bringing in other characters from the video game world.
Most of the people, especially the ones who grew up playing Nintendo consoles, may have heard about the Super Smash Bros. series. For those who haven't heard about it, Super Smash Bros. is a rather unique fighting game that is different from your typical fighting games and the brainchild of series creator Masahiro Sakurai.
For starters, it is an unorthodox fighting game where instead of attacks depleting a health bar, attacks knock players off a centralized platform. If a player is knocked far enough from a stage, they are KO'ed and lose a life, until one player or team remains. Also, unlike other fighting games, where characters have a specific combos depending on the button input, in Smash, players can make their own unique combos based on each character's default moveset. Another unique aspect of the series, especially in the Wii U and Ultimate version, is that third party characters are included for the first time in the series, therefore diversifying fanbase and playerbase alike.
For Smash Ultimate, what sets it apart from other crossover fighting games is the inclusion of a variety of video game characters not just from Nintendo games, but also third-party games. Nintendo knows the demographics in terms of what characters the gamers want so they tend to handpick some fan-favorite yet well-known characters. Characters such as Terry Bogard from Fatal Fury, Steve from Minecraft, and Min-Min from ARMS are few of the many third-party characters that were included as DLC's (Downloadable Characters).
However, it's not just the characters from today's video game that were included as DLC's but also characters that date back as early as the 90's that were included in today's Smash. Simply put, Nintendo's empire is built on nostalgia and they know that classic gamers will be hyped up yet feeling really nostalgic when they see their retro characters being included in Smash.
The video above depicts one of the many fighter reveals by Nintendo, and in this case, it's Terry from Fatal Fury, a fighting game that dates back to the 90's. Nintendo knows that people have certain strong attraction and association to said characters and the company uses this as their main driving force. Nintendo utilized various marketing channels, mainly print and digital, and releases announcements in irregular intervals via Nintendo Direct. All Nintendo has to do is to tease and get the fans hyped up, just like the reveal trailer depicted above, and the reason why Nintendo picked Terry out of the few DLC's are that Terry is the poster child of the Fatal Fury series, therefore he is loved by many of the players, as well as for nostalgia purposes, and the way the video led to the fighter reveal is just pure hype. Fans simply cannot wait to try out the new character whenever Nintendo announces the release date for said new character. For every announcement trailer for each new fighter, Nintendo does the trailer in a way to keep the theme and background specifically in-line for characters that will appear in the game in the near future.
If you have already watched Terry's reveal trailer then you will definitely notice the difference in the characters' themes, but the same principle of hyping up the crowd still stands.
As of today, Smash Ultimate currently has 80 fighters from different video game universes, with 2 more being added in the near future. Through introducing third party characters outside of Nintendo and constantly hyping up fans, new and existing alike, Nintendo's unique global marketing strategy continues to introduce newer players to Smash Ultimate while expanding the fanbase.