The Byzantine-Norman Wars that took place in the 11th and 12 centuries were an exhaustive, bloody process that wound up draining both powers of desperately needed forces. Deaths of key leaders, German & Venetian involvement and political infighting on both sides no doubt played a part in prolonging the conflict.
But, suppose the Normans were able to carve out key early victories under Robert Guiscard, enough so that over time, portions of Greece proper joined the Norman dominions in Sicily and Southern Italy. Enough, that rather than be King of Sicily, the Normans (and any successors) are crowned as Kings of Greece.
Thoughts?