I don’t understand the fixation people have on this character. With that said I do think that he’ll be getting a spot on the roster. With all of these fan favorites getting in I think it’s inevitable, but I wanna hear why he deserves it. He was in a spin off Mario RPG game years and years ago and hasn’t been relevant since. Why does he deserve the spot I think he’ll probably be getting? That’s my opinion, I just don’t care about him. Why should I?
A lot of it comes down to several factors. He's always been a popular character with people that like the Mario series. He is such an anomaly that doesn't really fit this Mario character archetype that he just stands out more as opposed to a character like Bowser Jr or even Waluigi. Even though it doesn't seem like it, Geno has always been a popular character. It was his cult status that got him to appear as a cameo in M&L: Superstar Saga. He may be a one time character, but it isn't like you see droves of support and fan art for characters like Mouser or Tatanga. Just look at some of the Newground videos. Some of them date back as far as 2007 with Rawest Forest being one of the most recognizable Brawl era animation for Mario RPG.
There is also just how well he can fit into Smash Bros. His moveset has a lot of flare to it and speaks for itself. I run into a lot of people who have support for another character but they can never truly vision exactly what they could do aside from 'it would be cool to have him/her'
Another note is how fond Sakurai has always been for the character. He has been on record talking about Geno since 2006. So you have that crowd of players who want to see Sakurai finally get his character.
The one thing I don't see a lot of people give credit to Super Mario RPG is how it defined the JRPG genre in the west. It was an introduction to these type of games for a lot of players and it even took steps forward to actually give us action RPGs whether they be Paper Mario or even Kingdom Hearts. Without the success and charm of this game, JRPGs might not have been able to take off in the west. So a lot of people view Geno as a symbolic character for Super Mario RPG as a whole.
His lack of being involved in other games is not his fault. It is solely on the hands of Square Enix and their overall unexplained stinginess towards the series. There was a time SE had a chance to feature Geno in a couple of spin off Mario games like Sports Mix and Fortune Street, but they instead gave us unfitting characters like Cactuar and Moogle. I guarantee if he was under the Nintendo banner, he would be in as many spin off games as a character like Waluigi.
Even if Mario is the titular character, Super Mario RPG is known more to be Geno's game. It's kind of like how 'Mario Is Missing' is for Luigi for Mansion (bad example but let's just go with that)
There is a lot of appeal for Geno for more than one way and it has been going strong and growing since around the time Melee came out. I pretty much am fully under the belief that "No character deserves a spot in Smash Bros. But with that being said, Geno deserves a spot in Smash Bros."
There is just a lot of reasons. (I think I got them all)