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!