During the Regin of William the 'Good' his forces attacked and invaded the Byzantine Balkans, with over 80,000 troops and 200 ships his forces amrched from Eprius to Thessaloniki successfully putting to flight any Byzantine Army that attacked them. In 1185 it was only with the defeat of William's forces by Alexios Branas on behalf of the Soon-To-Be Emperor Issac II Angelos that the Norman-Sicilians were defeated and allowed to leave giving up all gains in the Balkans.
What if instead of this fateful battle on the Struma river, William's forces defeats the Byzantines?
I think it largely depends on if William's commander in charge is able to absolutely defeat Alexios Brenas, then more then likely the Byzantine Empire may just start to crumble with rivals, the Bulgarians may still establish their Second Empire, and no alliance bewteen the Byzantines and Conrad of Montferrat may not end up (therefore he may not take rule as King of Jeruselum).