Answer: He wanted to set up Constantinople as his own capital city.
Explanation: Because it would allowed the Ottomans to more effectively invade mainland Europe, eventually leading to Ottoman control of much of the Balkan peninsula, because of position (its on Europe and Asia), and because of cultural importance, and at that time, he's on the most important trade route.
Extra explanation: He wanted also to set up Constantinople as his own capital city because it would marked final end of the Roman Empire, a state which dated back to 27 BC and lasted nearly 1,500 years.