i love it! nice idea! here's some suggestions like u wanted
music - definitely use the boss theme frome sonic 3
sound fx - some laser sound and exploding sound. maybe a booster sound for his shoes
enemies - the flying robot from sky sanctuary in sonic and knuckles. u can use that as a boss. also maybe some other flying robots like that bug thing in sonic one
powerups - invincibility (in a box of course!) rings (collect all to get an emerald after the level, not in boxes) superlaser (power up laser for a short period of time. this means non-stop laser. not in boxes) shell like from sonic 1 and 2 (protect sonic from one attack. in box)
other - ability to use super sonic (only availible after all 7 emeralds found. use for a very short period of time.) call help (sonic calls knuckles or tails for help) other characters (use them all in different levels!)
cant wait for the full version!