Geno is the love child of two of the greatest gaming companies ever. (I have to say this too often) Simply put, Geno represents the relationship and achievements Ninty and SE have accomplished together. I don't think you can say the same for Slime or Black Mage.
Pretty much you had Nintendo at its' peak, and Square Enix at its' peak, working on a game together.
Nintendo had recently made or was in the process of making classics like Super Mario All-Stars, Super Mario World 1&2, Super Mario Land 1&2, Donkey Kong Country 1-3, A Link to the Past, Link's Awakening, Super Metroid, Kirby's Dreamland 1-3, Star Fox, F-Zero, EarthBound, and Fire Emblem 3-5.
Square Soft had just made Final Fantasy III, Final Fantasy IV, Final Fantasy V, Final Fantasy VI, Chrono Trigger, Secret of Mana, and Radical Dreamers.
It was simply, at the time, a match made in heaven. Both companies constantly thrive to reach the creative genius at that level. Both companies were pushing and setting the boundaries like few other video game companies in the history of the industry. Both companies kept getting better and better, and improving everything, while still creating new, original, long-lasting series (for Nintendo, F-Zero and Donkey Kong Country, and for Square Soft, Secret of Mana and Chrono Trigger). Neither companies have created classic franchises and improved classic franchises at the same time quite like them. That's where part of the nostalgia comes from.
Also, I would hate to burst anyone's bubble, but Star pointed out that Brawl's questionaire is probably going to play a part in deciding who's in the next game. This was taken when Geno fans were at their peak. So, go figure~
I hate to burst your bubble, but Square Enix can be a legal hassle. Fortunately, Nintendo seems to understand them well. Still, Geno is definitely in Sakurai's mind as a potential newcomer. Then again, probably at least 100 characters...
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