Much like Ridley and King K Rool who are highly requested characters, Geno is one of them that they want him ever since he first appeared in Super Mario RPG: Legend of the seven stars. When i first heard of Geno many years ago i wasn't very familiar with him,cause i never played the game. The popularity for Geno has grown and fans still demand him to add in Super smash bros ultimate. Masahiro Sakurai said "Geno was actually a character I wanted to include as a fighter. He has a gun for an arm, and just seems like he'd fit absolutely perfectly into Smash. I was hoping I'd be able to put him in as far back as Super Smash Bros. Brawl, but unfortunately that never ended up materializing."
Due to Square-Enix's relationship with Nintendo we reunited after they broke up back in the late 90's to work games that plays on CDs. And also that Square created Super Mario RPG and owns the characters that they created such as Geno, Mallow and others from Super Mario RPG. Now the relationship grew and also for including Cloud from Final Fantasy 7 in smash for 3DS/Wii U and Geno as a mii costume for the mii gunner.
And heck some characters that they wanted in smash bros such as Inklings, Chrom and King K Rool were mii costumes and then later appeared as playable characters in ultimate, now it looks like that the doll will have a chance to appear as a playable fighter.