If we're thinking Dixie can have DK's Up B to be a Diddy echo, then I'm wondering if Tails could be a Sonic echo by stealing the same move? Heck, you wouldn't even need to do that to appear canonical as Tails does use spring pads but the DK Up Special would at least attempt to incorporate Tails' flight. We also know they are willing to reanimate attacks, which means Tails' tails can be shown off in the Smash attacks like they are in Sonic. And other than the tails, Sonic and Tails are pretty similar in build. Not 1:1 like Sonic and Shadow, but the differences aren't substantial. Tails is shorter (only by 6 inches in Sonic canon, which doesn't mean much of anything in Smash), and echoes can be a bit shorter in terms of a model (see Lucina and Daisy).
idk, I think it'd be funner to see Tails instead of Shad-ow the edge-hog. No offense to Shadow fans.
I feel like Tails was also a higher ballot pick than Shadow given that Tails got a Mii Costume (for a gunner no less, which could've fit Shadow perfectly), while Shadow got nothing.