Here's what Geno could make his appearance in Super Smash Bros. Emerald (and Super Smash Bros. Worlds Collide) instead of a random troll character like the Excitebiker. He's an unlockable fighter.
All of his moves are obviously taken from Super Mario RPG, however a lot of his attacks, especially his aerials, are completely made up, but also keeping his personality true to its source material. Geno has special team-up names for Mario, Peach, Bowser and Rosalina as well, as a nod to his debut where they were all part of the party for the former four and Rosalina's ties to the cosmos.
For Japanese dialogue, Geno is voiced by KENN, the same who voiced Jaden Yuki (Yu-Gi-Oh! GX), Apollo Justice (Ace Attorney) and Patroklos (Soulcalibur VI). For English dialogue, he's voiced by Yuri Lowenthal, who previously voiced Ben 10 from 2008-2013 and also voiced Derek Stiles (Trauma Center series).
Geno's stage is Bowser's Keep from Super Mario RPG. This is the first stage in Smash to take place in one of Bowser's Castles, and is rather simple. You start off fighting on top of the chandeliers just like the Bowser fight from Super Mario RPG, with black space surrounding it. The chandeliers are held up by a chain known as a Kinklink. Both chandeliers one Kinklink attached to them. By attacking a Kinklink, it will eventually break off. Once you have defeated a Kinklink from either one of the chandeliers, the chandelier will fall for a great distance, referencing the scene where Mario and Bowser fall for a considerable amount of time in the original game after defeating Bowser in a boss fight. Eventually, you will arrive at a random section of the Keep, with each section being different in layout and design. Plenty of cameos make appearances as well, such as Mallow, Frogfucius, Booster, Croco, Jonathan Jones, and various enemies from Super Mario RPG.
After a while, the Chandelier will repair itself after a little while and it will rise all the way up to the top of the Keep again, reconnecting itself with the Kinklink.