On the topic of Sonic's moves being "too goofy" for Shadow, a reminder that Daisy has never been depicted with the ability to float in a Mario game, nor do the Toads serve her (Toads are residents of Mushroom Kingdom, Toads are not in Sarasaland), yet she became a 1:1 clone of Peach anyway with her floating and with her own Toads. When it comes to deciding clones, the most important thing is "can this character mechanically work as a clone of this other preexisting playable character?". Everything else such as source accuracy is completely secondary. Ganondorf became a clone of Captain Falcon in Melee, because he shared the same body build of a tall lean muscular man, thus you could map Ganondorf to Captain Falcon's skeleton and have him do all of Captain Falcon's moves without any technical issues.
When it comes to Shadow, you can make him a 1:1 clone Sonic without any technical issues as they're built pretty much exactly the same, and thus it remains a real possibility. Now if Sega says no to it is an entirely different matter, but as long as it has Sega's approval, Sakurai can make Shadow as much of a clone as he wants.