SonicMario
Smash Master
As someone heavy into the Sonic series. Yeah there's some Shadow fans that are conflicted on the idea of Shadow being an echo. But taking Dark Samus (Who never fought like Samus does at all) into account, Shadow still makes a whole lot of sense. And arguably more so then Dark Samus did because there are games Shadow practically played no different from Sonic did. And you could always pull a Chrom and give Shadow a teleport similar to Mewtwo's to replace the spring as well as make all of the dodge animations warp-like animations as well.As for Sonic- it's not quite my series so I'd go with the obvious choice and say Shadow. I don't think it's what anyone really wants (Shadow fans would want him to have Chaos Control powers of teleportation and the like, right?), but I think it's most likely. Though for a more interesting choice... Mighty the Armadillo?
Not to sound rude, as that's certainly not my intention, but I'm curious as to how you see this working? Again, Sonic isn't my series so I don't know that much about Tails, but isn't he more of a gadgeteer as opposed to Sonic's Speedster?
As for Tails. In the Classic games, he wasn't too different from Sonic beyond flight ability in Sonic 3 (& Knuckles). However that flight ability is so much more significant of a difference. You can make Shadow play like Sonic and still make him feel like Shadow, but Tails without flight nowadays (While in his debut game if you played as Tails only you could not fly. Pardon the pun, but I don't think you could get away with it as it wouldn't fly today) just would not be Tails. And unlike Mewtwo's teleport there isn't a recovery move that is like Tails' flight in the game any more. The closest Smash Bros. had to a more Tails-like flight in the series is Pit... back in Brawl. And that recovery move was replaced. Unless for some reason they brought that move back just to give an echo Tails one original move (Even if based on an old move)
Dixie's flight ability was enough to make Dixie's echo candidacy here split and even slightly in favor being not an echo. The same should be said for Tails.
I'd take Tails if he had to be an echo to get in, but I currently see no reason why he couldn't be a semi-clone in DLC and possibly even packaged with Shadow as a Sonic echo assuming Shadow's not in the base game.