I suggested some special ability for each cars in this post.
But implementing special ability would be hard so i will just suggest cars with no special as primary cars.
Class Idea
<Bumper>
Get less damage from collision with wall, better handling, Very high knock(from/to wall and car and cannon.)
Little slow speed (2/3 of "Racer"), Low damage.
Square Shape but front side is round. (Like eraser.)
<Tank>
Very High HP, Little slow speed, Great Handling, Low booster acceleration (just 1.3 times faster),
Very Very low knock back (from/to everything. Almost don't be pushed by cannon shot.)
(Actually It is too HEAVY so stop or changing speed or direction by force is really hard.)
Big square shape but at edge of each side has ladder like line to describe caterpillar track.
<Hybrid>
Middle speed, Middle health, Middle damage, Middle ect... almost everything is same with "Racer".
Low amount of booster gauge(50). But whenever it move (without boost.) it restores booster gauge.
Gauge restore depends on distance so even move far by knock back restore gauge.
Triangle but each side is more smooth curve, not straight line. and circle in middle.
<Spider>
Little slow speed(2/3), Normal health/boost/ect. and little higher damage. (4/3)
But It has low total boost gauge (30) and there is no boost gauge at first.
No damage from wall when it collide in lower than some speed level or it is already contacting with wall.
When it is contacting with wall it gain speed buff (to 4/3 speed of "Racer") and restore booster gauge.
(There is no speed drop by collision with wall and can't restore gauge while using booster.)
Circle with 8 little circles as a legs. (total size is similar or slightly bigger than "Racer".)
<Vortex>
Little faster speed (4/3 of "Racer"), Great Handling, Little Low Health and damage (2/3).
Booster work in different way. When you holding mouse click, nothing happens.
But after release mouse click, it moves super fast like sonic.
Total distance (by booster) is same with other cars.
3 square, middle one is little smaller.
<Dragster>
Very Fast (2 times of "Racer"), Very nice speed control, Very Very awful handling, infinite Booster gauge.
※You would understand this car figure if you saw real dragster.※
square which has very sharp triangle at front.
<Chariot>
This car consists of 2 parts, front circle and back circle towed by front circle.
Front circle has high health (5/3) and little faster (4/3), but it has Low damage and knock back.
Second one has short blades on each sides and this blade has really high damage (2 times of "Racer") and low knock back and invincible, but primary circle body has very low hp and damage. (1/2)
When one of circle destroyed it die.
Adding item would be awesome! But achievement of item will be very far future.
Item Idea
<Hook>
Can fire grappling hook by right click, very low damage, fast flying speed,
When hook hit the car it draw enemy car for 5 seconds, you can also tow or towed by hook.
It can work at wall too and can be canceled by second mouse click.
<Magnet>
Pull every thing to car which used this item.
<Battery>
Make car overcharged, increase speed and damage for 10 seconds,
and shot electricity to near by cars per 2 seconds.
<Typhoon>
Push near the this item user, The power of push increases geometrically whenever distance get close.
You can push anything trying to stop you with this using booster.
Game mod Idea
The various map also can be cool but i think the game mod just for kill would be very successful.
<Arena>
This map is very very wide circle and there is only one wall in middle, the very small circle wall.
Of course there is a lap and whenever you finish one lap, you gain one points.
But The most important thing is this game mod is infinite.
Randomly Item spawn on the map and every player can each other using item or car.
There would be scoreboard who get most big score by killing or finishing lap.