Shadow's an interesting case. He can easily be an echo, but it's awkward not having his chaos powers. However, he still has used them quite often more like a monk fighting than just things like projectiles and energy waves. Similar to how Dark Samus is, you can add Shadow's chaos stuff through animations. It's more awkward for him to punch instead of use chops, honestly. He's not a puncher as is. Sonic's moveset can easily be used, but it depends how many animations need changing before they feel they might as well go the whole route.
Whether he's an echo or not won't depend really much on "moveset potential", it'll depend upon how much work is required to get him feeling faithful enough in Sakurai's opinion(as well as what Sega wants, which is probably a part of why they aren't sure he's worth the hassle of licensing). That's not even counting costume issues either. We don't really know how Sega even feels about getting a second Sonic character. No matter who asks, they need to actually say yes. Would they say no? I doubt it, but that's always a possibility. Shadow might've been an echo candidate.
Also, I want to note that Dixie doesn't really "Float" so much as use her hair like a copter. Besides that, her hair abilities are fairly ease to recreate without even making her a clone or semi-clone. A key thing to note is that she just needs someone with a similar bodyframe to rip animations from/retool them. DK's own spin fills that mark, since they actually do have similar skeletons. Coincidentally Diddy no longer has his tail moves bar a get up attack, which is really easy to retool.
I'd rather have both as unique, but both can actually work as Echoes. Among the ones that were heavily discussed, Isabelle was the only one not possible to be an Echo outright that we know from experience. You can apply that to most Sonic characters(Shadow and Metal Sonic are the only ones with the actual same bodyshape), for an echo of Sonic.