Something that just occurred to me in regards to 3rd party echo fighters. Before I begin, i'm not inherently against 3rd Party Echo Fighters. The only reason I'm talking about it is because I think their likelihood is way overblown. I'll explain why.
First, what's the way by which echo fighters are added? Like clones before them, these fighters are typically added at the end of development when there's a bit of time left. These are basically the "cherry on top" of the roster, and typically satisfy the wants of fans whose characters wouldn't necessarily be added first in line. This is exactly how the clones of Melee were put into the game, as well as Smash 4.
Next, there's absolutely no indication that Daisy was added in a different way than Lucina or Dark Pit were in Smash Wii U. As she is branded the same way that they are, there's frankly no reason to believe she was planned as anything other than a last minute "cherry on top". Was she a focus of the Direct? Yes. But that reveal served more of a purpose than just the character herself.
In addition, 3rd party contracts are typically put into effect at the beginning of development and based on that agreement basically guarantees the addition and development of the character. If Echo fighters are last minute additions that can be dragged and dropped with minimal changes, then that would mean the contract for the Echo fighter to be created would have to be determined at the end of development, which usually coincides with balancing.
While I will not try to say I know how these businesses work together, I will say that there are several things in the business world that take more than a day...even the simplest things. I don't think it's impossible for a 3rd party like Shadow or Ken to be added as an Echo Fighter, but I sincerely do not think they're likely in the slightest.
Youtubers and users on all sorts of different websites are now all of a sudden predicting characters like Shadow, and there's honestly no logical reason to believe as of right now that they're any more likely than before. Clones/Echo Fighters have always been added as last minute additions, and they're basically the leftovers.
Could Shadow be planned in terms of a pool for Echo Fighters at the end? Sure. But then that would add uncertainty to a potential contract with SEGA, as the timing would have to be just right. If you're going in on a contract, you want to for sure commit to it rather than, "Oh yeah, we've got enough time to add this character that doesn't belong to us". That's not how it would work. SEGA would absolutely want their character in the game if they were signing off on allowing them to be there.
Basically, chances of echo fighters assumes any and all potential characters weren't planned for something else first or that any legal considerations were in place prior to their planned inclusion. This gets complicated with involving other companies in the process of making characters that are based on meeting popular requests while also being easy to obtain and make.