The style is great, and the game looks really original !
It could be selled as a lot of little games are.

For levels cartoon style , you could use birghter colors , but
that don't are too ocntrasted.
Best example take a look at
the game Ratchet And Clank on PS2 , Or Wind Walker, look at their colors used on textures.

But perhaps it's the style of the game ?

Another key you oculd use is trying t odiversify gameplay, beat em up on one
level , treasure serach in another , some simple arcade racing on another in a
straight line track if you don't have tools/time and all
to make curved roads ?

Keep it up you have something good , original
even if i don't have played it for the moment