Regardless of whether Sora is an impediment to Geno, we need another anime swordsman like we need a hole in our collective head. What with Terry around, it's a moot point to make but I'll ask it anyway: what would Sakurai see in Sora aside from a way to attract KH fans, who would likely never enjoy Smash because it involves hitting more than one button every fight. What does Sora bring to the table that Hero or Robin hasn't already? Air combos? Bayonetta covers that niche nicely. All he would be is a huge licensing hassle to bring what would basically be another Joker to the game - a character with only spinoffs on Nintendo consoles while the mainline entries stay nearly exclusive to Sony.
Hmm...
Didn't Hero have huge licensing issues too...
Also, your lack of Kingdom Hearts knowledge is showing. As a resident expert (unfortunately) let me inform you that every single KH game is canon and part of the main lore. Of those on Nintendo system, the game that explains what happens between KH1 and KH2 was on the Gameboy Advance; the game that explained the story of what ended up being three of the most popular characters in the series was on the Nintendo DS (and is, to date, the only KH game to feature multiplayer in any form); and the game that explains what happened between KH2 and KH3 as far as Sora's story is concerned was on the 3DS. Sure, these games have either been remade or have been re-released/their story explained on other consoles, but they originated on Nintendo consoles.
Let me be the first to tell you that, by technicality, Sora is not an anime swordsman...he's an anime KEYBLADESman. Jokes and anal-lytics aside, this is a very big difference as it puts Sora in a situation more akin to typical JRPG characters rather than Fire Emblem: having physical skills, spells, summons, items, etc.
By the by, saying that a character already in the game offers something and thus another character who offers that is pointless is not a good argument. What if I said the Dante offers nothing Bayonetta already does because of her combo structure and how she already represents the hack-n-slash genre. Not only does the statement mean nothing and hold no weight as we've already seen similar characters come in and offer more, but it's an insult to the speculated character. But, since you INSIST on focusing on what Sora could bring...
Imagine Hero...but with less spells that aren't as powerful and better normals and aerials that offers combo potential similar to Bayonetta. That right there is something already not in Smash: an 'anime swordsman' with spells AND natural flowing combo structure. Would be pretty crazy with just this, honestly.
However, for some more uniqueness, KH2 offers Drive Forms. Were Sora to have Donald and Goofy as a part of his moveset and have Goofy and Donald function somewhat independently from Sora like his games (think Ice Climbers, Konohamaru Ninja Squad from Naruto: Ultimate Ninja 3, or Gargos from Killer Instinct for Xbone), Sora could temporarily sacrifice them to obtain a more powerful form that would change his gameplay. Sacrificing Goofy gives Sora two keyblades, major damage potential/increase, a new combo structure and armor on some moves but locks him out of using Magic. Sacrificing Donald would give him Wisdom Form, which would buff his Magic and cause him to be much more ranged-focused but he would lose his CQC combo structure. Sacrificing both would trigger Master Form which would incorporate Magic into his physical attacks and alter his spells and combo structure but not have him excel in in either range or CQC while also taking more damage. Based off RNG, sacrificing both could trigger Final Form, which would be a much more powerful version of Master Form but would be even more glass canon. Spamming Drive Form increases the chance of triggering Anti-Form which deals little damage, takes heavy damage, and removes both Donald and Goofy. Would also have a cooldown between activations to simulate how it works in KH2 with having to build the Drive Gauge.
For something more recent, KH3 introduced Keyblade transformations. I have no idea how they would incorporate this, but having a transforming weapon would surely offer unique gameplay. They could even incorporate Flow Motion which would give Sora crazy movement options whenever a wall is present that can also be attacks.
This is all off-the-cuff, but I think you get the idea. Sora would be an amazing inclusion and offer much to Smash.