While I will say that Tails should have higher priority, Dark Samus's playable inclusion could mean something for Shadow. However, I don't see Shadow being an Echo Fighter, but more of a semi-clone who copies some of Sonic's attacks, but also references a couple other fighters.
For one example, Shadow's d-tilt would have him sliding forward, similar to Cloud's d-tilt.
For the specials, I was thinking the following...
neutral - Chaos Spear: Shadow charges up an energy spear, and fires it at his target. When fully charged, the spear will split into multiple spears, and home in on the closest opponent. On a side note, Shadow can hold the charge.
side - Spin Dash: Shadow curls up into a ball and dashes forward. Unlike Sonic's Spin Dash, Shadow does not jump, and the attack hits repeatedly. Shadow also receives damage-based heavy armor during the attack, making him impervious to flinching against attacks that do not deal more than 8% damage.
up - Chaos Control: Shadow uses a Chaos Emerald to teleport to another area. The move does not leave Shadow helpless, but it does prevent him from using any special moves until he touches ground. Also, Shadow can draw in nearby fighters with this move, causing them to teleport alongside him (sort of like a grab move). But upon reappearing, the fighters will take some damage, and be sent flying.
down - Chaos Blast: Shadow creates an explosion around him that deals damage to nearby opponents. The attack can be charged, which increases the blast radius, along with how frequently the explosion will hit its targets. The full charge explosion deals up to 16 hits, and deals a grand total of 20% damage; the final hit deals the most knockback.
As for the Final Smash...
Super Shadow: Shadow changes into his super form, and dashes forward. If he hits anyone, they'll be brought over to the highest point of a stage, and Shadow will start dashing around the fighters that he hit to build up their damage. Shadow will then finish up with a powered up karate chop that sends fighters back down to the ground, and upon touching ground, he'll generate an explosion that sends all affected fighters flying with high amounts of knockback taken. This Final Smash operates in a similar manner as Ike's Great Aether and Cloud's Omnislash.