I'm liking this game idea! All of your designs and models look very nice and fitting to the style.

I will literally sit here all day if you ask me to make a story behind Turbo Rabbit, there are so many ideas. You can ask yourself: Why do the islands have gems? Where did they come from? What are the consequences of taking these gems from these islands? Why does Michael have turbo shoes? Did he make them? Did he find them? Was he born with them? Did someone give them to him?

What about the monsters? How did they get to the islands? Why are they there? Why do they hurt Michael? Are they evil? Are they guarding the gems? 

I could go on, but you get the point. Asking yourself questions and coming up with answers can help you come up with a story!

I really like everything you've made! You're very talented!   

It looks like what a game combining Mario and Sonic would feel like.

Sounds like basic setup for a game. But it's good thing cause it makes story creation easier.

You can have main story either:

A. Michael is tasked by superior being to collect the gems for good purpose. However everytime Michael is about to take a gem, a mysterious voice warns him not to. Michael ignored the voice and resume the collection. After all gems are collected, the being used the gems to bring back his lost power and regain his status as evil emperor of the world. It turns out the gems were his source of power which were taken from him to weaken him and seal him 1000 years ago (cliche).

or

B. An evil group is collecting the gems for their evil purpose. Michael thinks that the best way to stop them is to collect the gems earlier than them. This strategy works except the general of the group realized this and steal all the gems after Michael collected all gems. Michael has to go to their base to get them back.

Which one do you like?

I'm aware those two stories are pretty cliche but we could add twists later if needed ;).