Sakurai made alot of References to Ken in Smash 4 before he got into Base game
The Same was done with Sephiroth in Smash 4, before being DLC in this game
I think with Certain Characters, Like Dante, If they get into the Roster (eventually) I expect their Rival or Equal to Show Up (Vergil) eventually as well
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Regarding Sonic, I Imagine that if they were able to get Sephiroth in the game, There shouldn't be any issue attaining a 2nd Character in this game. Sega loves throwing Sonic and Other characters around in Crossovers. I don't think Smash is an exception to this rule
I do understand that people do throw around the idea due to How Sega might mandate Characters (Sonic's Alts only being Shades of Blue, How they intially turned him down in Brawl). There's also the sentiment that people thought we would have gotten Shadow as echo if there wasn't something Terrible that happened in Negotation process (or another Character like Tails or Knuckles)
Neither Tails, Knuckles, or Shadow work as Echoes when considering Smash. for Shadow in Particular, Shadow has been a Mirror of Sonic Gameplay wise in good amount of the Sonic Games. with that being said, if you Translate that to Smash, the only thing Majorly Shared is the Spin Dash and Homing Attack. Shadow's Whole Animation and Demeanor is Completely Different from Sonic even in Games where they play the Same. This also isn't taking into account the Boss fight, Cutscenes, Lore and the games where Shadow's Moveset is More unique. With Sega having to say yes to Everything regarding Shadow's Hypothetical moveset, There's no way Shadow would have gotten away with being an Echo
I think if Sephiroth majorly benefitted from being DLC Now (Due to the FF7 Remake and such), than Another Sonic character benefits because the 30th Anniversary is coming in June of 2021, and The Movie came out this year
Anniversaries I do think might be overlooked in terms of importance. People go "Anniversaries mean nothing". I think that dubious. A Character or Franchise having an Anniversary does not guarantee a character being in yes, but It does add to their merits, because Companies like to go all out for Anniversaries and such. Sega is a Very good example since they always release Sonic games on Anniversaries (Even to a Fault). Nintendo would likely know it's coming up and will know that Sonic will be very Relevant going into 2021
Sephiroth's Presentation is on the Anniversary of the Franchise (in Japan), I Believe Minecon is held around the Anniversary of Minecraft (I Don't know about this one), and Min-Min was fully revealed in the Same month as the Anniversary
Having said all of that, Another Sonic Character isn't guaranteed. As Much as I Might like to say that Shadow will be in this Fighter pass, No it isn't a Guarantee nor Confirmed, Same with Eggman, Tails, or Knuckles. I also don't think Shadow or Knuckles should be Slept on because they are Assists. I can understand more Apprehension because they already have that Role, but they Both are Still Important and Very Popular Sonic characters and Iconic Characters in General, and if you believe in the Idea of an Assist Promotion, both these characters should be on your Radar