Knuckles and Tails I approve of.Of course like most people I want people like Tom Nook, and all the other characters that people want like megaman, but I definitely think they need to expand sonic, they should add tails, knuckles, robotnik, and amy, but the reason I am posting is because there should be NO SHADOW. Shadow was not invented till freakin' gamecube, he is not at all a classic sega or nintendo character, he is some modern reincarnation of a sonic villain which is stupid, he is not classic and would suck in brawl. It is simply a stupid idea. Other good sonic characters would be big the cat, antoine d'coolette (he gots a sword), and bunny rabbot who is half robot. The characters final smashes could be just like the ideo games, tails had flying pirhanas that circled him and ate anyone near him, and knuckles turned into hyper knuckles and was hot pink. Robotnik could get into one of his pimped out robots. Just my ideas.
Amy had no gameplay element besides harassing sonic with love until Dreamcast.
Big has...a fishing pole.
Bunny is a SatAM character, not a game character.
Tails had Yellow Flickies surrounding him(I assume you mean the S3&K S/H Tails)
Knuckles' Hyper form was just a glorified Invincibility. Make his Final smash energy extensions of his punches, or some sort of unlimited flying/gliding mode.
Robotnik is pretty versatile. He'd make a decent reoccurring/multiform enemy.