Making the Drift tighter when using the Boost.
Mapping the Homing Attack to A. X would result in an Air Dash instead before acquiring the Air Boost Shoes.
Fun Skills for Sonic bought with currency by doing missions from townspeople. Some examples...
Infinite Boost:
Sonic can Boost infinitely at the cost of dying in one hit. Perfect for speedruns.
Spindrift:
Drifting builds up power and stopping releases it. When the power is released, Sonic is bursted forward. (Yes, this takes inspiration from Mario Kart.)
Double Jump:
Sonic will Double Jump instead of Air Dash. Useful for platforming.
Badnik Bonus:
Defeating enemies rewards Sonic with Ring Energy.
Blast Off:
Starting a Boost as soon as the stage starts results in Sonic blasting forward even faster.
Strong-Footed:
Sonic loses less speed going up inclines and gains more going down inclines.
Power Break:
Sonic skids to a halt much quicker.
Ledge Catch:
Sonic will catch himself when he's on the tip of a ledge when pulling away. He'll lose all his speed and continue running afterwards.
Power Stomp:
Sonic will send shockwaves around him when landing from a Stomp.
Spindash:
Sonic can use his trusty Spindash by holding down B and releasing it. In the air, Sonic can hold B to charge a Dropdash that releases automatically upon touching the ground. The Spindash can provide even more extra speed when used correctly.
Squat Leap: Sonic can jump higher when jumping from a squat. This combined with the Air Boost is literally just platforming cheese, lol.
Bolt of Light:
Sonic doesn't damage enemies as he Boosts, but he goes much faster. Use the Slide to tear through enemies or avoid them if possible.
Anyways the upgrades would be categorized as skills as well. You can equip more skills at once by upgrading Sonic.
That's about it besides the obligatory "More Content!" Everybody always wants more stages to play, y'know.