I disagree with your assessment of his popularity, Shadows popularity was a product of his time, with the whole edge thing. ( I feel the best way to fix shadow for a younger audience is to turn him into Reverse Flash or Eobard Thawne, but that's just me) aside from that, I agree wholeheartedly, please, show us your move set. ( I can't find a way to word that un-condescendingly, so just know I mean it genuinely)
Definitely a fair point, just taking into consideration of Episode Shadow from Sonic Forces where he is the only one of Sonic’s old friends to be playable and him hold down his Assist Trophy role from (at least) Brawl to Sm4sh, he seems to still be relevant. But of course to each his own. Personally I don’t care too much for Shadow, having a “meh” opinion of him.
As far as his moveset goes, I tried to keep what few "rules" we have about Echo Fighters in mind and attempted to not stray too far from Sonic as far as the actual moves go. However, as seen by Daisy, animations are free for altercation. But I feel this moveset would reflect Shadow quite well.
Attributes: Shadow appears more serious and stern than Sonic, being less arrogant and peppy. His neutral face is a serious frown unlike Sonic's smirk, his walk is less hoppy, different Spin Attack trails, etc. Shadow would share many attributes with Sonic like run speed and weight. One big difference both aesthetically and physically are his Air Shoes. Not only does he use them for his running animation, but also in his kicking attacks, giving them a fire effect and a little more knockback. However, these Air Shoe attacks come out a little slower and have a bit more cooldown. This would be a common theme throughout a lot of his attacks in comparison to Sonic, trading some speed for raw power, as true to the actual series.
Neutral Attack: Sonic's PPK except the final kick emits a flames from his Air Shoes.
Forward Tilt: Horse Kick with flame effect.
Up Tilt: A single high kick that has more knockback than Sonic's double high kick. (referencing Shadow Kick from Sonic Battle)
Down Tilt: Leg sweep with flame effect.
Dash Attack: Lunging kick with flame effect.
Forward Smash: A charged up punch enveloped in Chaos energy, giving it a bit more beef. Doesn't wind up in the cartoony fashion like Sonic and just pulls his fist back (referencing Chaos Upper from SB)
Up Smash: Same as Sonic.
Down Smash: Split kick with flame effect.
Neutral Aerial: Same as Sonic.
Forward Aerial: Same as Sonic.
Back Aerial: A hook kick with flame effect.
Up Aerial: Same as Sonic.
Down Aerial: A downwards kick using his Air Shoes to propel himself even faster and harder, spiking throughout the entire move but makes him likely to SD offstage.
Grabs/Pummel: All same as Sonic
Neutral Special: Shadow's Homing Attack works very similarly to Sonic's except that his has no recoil animation, allowing him to attack right after with an aerial (referencing Shadow's Homing Attack in Sonic 06)
Side Special: Sonic's 3rd Custom Down Special Burning Spin Dash.
Up Special: There are a few ideas for this one. Using Chaos Control to teleport in the same or similar trajectory as Sonic's Spring Jump is the ideal Up Special for Shadow. However if this is too "different" for Echo status, I'd suggest Shadow still using Sonic's animation, but instead using his Air Shoes to propel himself upwards, still having the same functionality of Spring Jump minus the Spring and its hitbox and possibly hitbox under his Air Shoes. And if this is too different, I'd suggest Spring Jump with a Yellow Spring with less vertical height.
Down Special: Sonic's 2nd Custom Down Special Auto-Spin Charge. I'd also suggest making it a single strong hit instead of a multi-hit attack.
Final Smash: There are many possibilities for Shadow's Final Smash. If he were to "clone" another Final Smash, I'd suggest Bayonettta's Infernal Climax. I see Shadow using a Chaos Emerald to use Chaos Control, slowing down time around the fighters. If enough damage and hits are done, the fighters caught in Shadow's final attack are sent into a cutscene of Shadow transforming into Super Shadow and using Chaos Blast to blast his opponents away. Or of course they could just carbon copy Super Sonic into Super Shadow.