I missed some Shadow Discussion, there are some things I need to add
-Since people like to mention how Japan presence is major factor into character's Inclusion, I have to say that the biggest Sonic games were SA1, SA2, Heroes, and Sonic 06
I think Sonic 1 and 2 did overall have the bigger sales, but the 4 other mentioned games resonated with Japanese audiences, which is something Sonic hasn't really done since. SA2 apparently is considered a Top Game by Famitsu readers and such especially. I have to really look into that though, but that's apparently what it is
SA2 is of course Shadow's first, and probably most Iconic appearance. SA2 is also the first game on Nintendo platforms, so this was the Sonic game that introduced Nintendo to Sonic and such.
-Shadow over Eggman and Tails is not something that is implausible. I do think people recognize that Shadow is more popular than Tails and Eggman, but this is honestly more impressive than people might realize
Eggman and Tails are Iconic characters in their own right, have a whole Decade over Shadow, including being in the Golden Age of Sonic (as some would say), Doesn't make as frequent of appearance as these 2, and are not as Associated with an era of Controversial games like Shadow is (with Shadow and 06)
Yet Shadow is still the most popular character after Sonic to this day, in spite of all of this. That is very impressive, and the reason this is achievable is that Shadow as a Character is strong. this is not to say Eggman and Tails are not good characters, but Shadow has resonated with more people overall
Plus Shadow is Head of Sonic's Team (Iizuka) favorite character, Perhaps Sakurai's Favorite character (though that is speculation), and Shadow has his own moves that need to be factored in when considering another character from Sonic.
-On the CGI trailer thing, I will say that unlike Gameplay portions of trailers, which uses the assets already made for the game, all the animations and expressions for the CGI need to be made specifically for the trailer, and I Imagine the 3rd party would need to be on board to clarify if the character is represented in the trailer well
Sakurai does have to contact 3rd parties when it comes to Content in Smash, he even had to do it to allow all music on FD/Battlefield stages. However Working with 3rd parties on CGI would be much more of process, since that is more hands on an not re-using assets already in game like with Gameplay footage and Music selection, So I think there is reason to believe that perhaps this does hint at something Sega related in the future coming
-I know Shadow is an Assist Trophy, and even among people that believe Assists could theoretically be promoted, people don't believe any of the Assist characters have reasons to be promoted to be taken seriously, This is honestly false
--People have to realize that if Shadow was added to the game, he'd be the most recognizable character added to DLC besides Steve. Sonic as a Brand and it's characters are some of the most recognizable in the industry
--Sonic has also been relevant, especially in the years of 2020 and 2021, and Nintendo would have known when finalizing the characters in FP1. The Movie was going to be coming out in 2020, and there is the knowledge that 2021 is Sonic's 30th, and especially since Sega always does something for the anniversaries, that is something to be factored in
--All of FP2 was finalized around November of 2019 or before, including CP11. The idea of an Assist being promoted in the last slot is more slimmer, but that goes for every character since there is one slot left. It doesn't change either way if the first slot was Assist promotion or even earlier slot, because everything was already decided in the beginning
--It's honestly more economical for Assist Promotion than not. Assists already have 3D assets in the game already, and Nintendo already knows the gist when it comes to negotiating for the Assist characters
Pretty much, Nintendo is familiar with the characters that are Assists, and I would say is less expensive to go with something familiar than it is unfamiliar, since there is less time and money that needs to be invested with something you have knowledge of in regards to the game. Nintendo is already well tuned with Sega, Is well tuned with Sonic, and is well tuned with Shadow specifically as well, in terms of the cost of the characters, how to represent them ingame, and such.
-Something regarding Shadow's Hypothetical moveset. Shadow would probably be the first True Snowball Character added to Smash if he was added
A Snowball character is someone/something that gets stronger the better they do in battle, through Mechanics and stuff. In Sonic games, Sonic and the Cast are allowed more opportunities for going Faster/getting upgrades if they do stuff like Get more Rings, Defeating opponents, and such
Shadow specifically follows this when he's been implemented as a Playable character. Chaos Spear and Chaos Control are moves you can use in SA2 Multiplayer if you get more Rings. In Shadow the Hedgehog, you got meter when Damaging or Killing enemies, and Same thing with Sonic Heroes and Sonic 06. Importantly in Sonic 06, unlike Sonic and Silver where their meter refills overtime, Shadow has to actively attack and Kill enemies to refill his Chaos Meter
This would not only be interesting from Gameplay standpoint, but It also would be good balancing in the case of Shadow, since Shadow's Toolset can easily make him overpowered if left unchecked