Alright, I think it's time for me, the resident diehard Sonic fanboy, to put this argument about Shadow to rest.
If Shadow were merely just an echo, it'd be a huge disappointment to me and many others. Although Shadow has the capability to be entirely unique, many would be happy if he was a Luigi-esque semi-clone. Anyways, at least one person apparently compared Shadow to Dark Pit, which is a huge mistake on their part: Dark Pit is literally a clone of Pit that has all of the same weapons, powers, abilities, strengths, and weaknesses as the original. Shadow was never a clone or anything of Sonic, he's just another hedgehog that happens to have a few similar abilities as Sonic, but has access to a variety of different powers and other things. Most notably, he has all of his Chaos Powers such as Spear, Control (for both slowing time and warping), and others seen in Sonic Battle including Nightmare, Burst, and Upper. Not to mention, he has the boosters in his shoes, meaning that kicking attacks can have an added flame effect, even if they're similar to Sonic's. He's called The Ultimate Lifeform instead of something like The Black Blur for a reason. I mean, sure, he can spin dash and use homing attacks, but doing exactly everything that Sonic does definitely seems wrong for him, personality and ability-wise. If anyone's a better echo of Sonic, it's Metal Sonic, who's literally a robotic copy of Sonic that's supposed to match everything that Sonic can do.
TLDR: Having Shadow as simply an echo would just be a complete disservice to him, when he's definitely capable to be unique and was never meant to be an exact copy of Sonic. If you want a completely unique character or a Luigi-esque semi-clone, then Shadow fits the bill. If you want an echo of Sonic, look no further than Metal Sonic, the Dark Pit to Sonic's Pit.