No way in hell will he go. Derek Fisher a very solid player in his own right would be valuable to anyones team, especially because of his Basketball IQ, however Fisher moved to LA for his daughter as she was extremely ill. Plus he is getting his minutes and is co captain of the Lakers. He is a very loyal guy as well and the Lakers just have as much chance if not more to win the Title.
Fisher wouldn't exactly fit as well in Miami anyways. He isn't as quick and would let to many quick point guards into Miamis somewhat weak 4 and 5 positions. He is in the Lakers because he is smart and knows what to do in every situation. However he would barely have the ball with Wade and Bron on the team, and then it would be sent into the post. Also Miamis Offense wouldn't fit into Fishers game.