Does your imagination become true if you say Inshallah?

No saying inshallah doesnt mean your wish will come true, the sentece inshallah means if god wills, so maybe god wont will you a specfic thing which you want.

And the reason we say inshallah is so that we dont make false promises like promising someone that you are going to do something but not beign able to do it later and in that case it was gods will to not happen.