The Byzantine Emperor John II Comnenus is considered by many scholars to be the most successful of his dynasty. He lead successful campaigns against the Pechenegs and Magyars, and re-took much of Asia Minor from the Seljuk Turks. He also campaigned in the Holy Land in cooperation with the Crusaders, and actually established Byzantine suzereignty over some of the Crusader States. His methodical approach to warfare conserved the Empire's forces and protected it's heartland, while slowly and steadily expanding it's territories.
He died prematurely in 1143 after being accidentally infected by a poisoned arrow while hunting. He was at the time on the verge of leading a new campaign in the Holy Land which might have made major gains for Christianity in that region. What if he had survived the wound, or the wound had never occurred, and John survived until 1160, dying at the ripe old age of 73? Let's assume also that his physical and mental powers remain strong until the last year of his life.
How might this have affected history and the future of the Byzantine Empire?