I'm sorry, but why must Shadow use a gun? Sure, he's edgy and he uses a gun in that one game, but I think his most interesting trait is that he can use Chaos powers without the need of all the emeralds. He should be a mix of Sonic and Mewtwo. And if anyone wonders how this may work, Sonic Battle for the GBA has you covered.
What are "chaos powers?" Vague energy blasts or something? And does he do that in any game that isn't Sonic Battle? The fact that he uses a gun in his very own game that his name is on the title of probably counts for something, particularly in a dry well of unique moveset potential.
If all they are are going to do is spin dash, then none (except for Dr. Robotnik)
The others have kind of drifted from doing that since the Genesis days, and none of them are as well known for it as Sonic, which is why Tails/Knuckles being echoes or semiclones would be odd.
Tails flies, has had a tail spin attack, had a mech walker, and used wacky gadgets in Sonic Battle. Which is really inconsistent and isn't a lot of moveset potential unless they went all into the gadgets thing employed in just that one game. He's really got nothing unique about him outside Battle.
Knuckles has plenty of unique abilities like climbing, gliding, and digging, on top of being a brawler, but like I said, to me having him playable but not Tails would be weird and incomplete.
Shadow's just Sonic with a gun and I guess you could put purple glowy chaos effects on his animations if you wanted to.
Eggman's got dozens of vehicles and gadgets from over the years to draw moves from in his egg carrier. Easily the most potential, I think.