Well, I am not an expert on the Crusading era, so hopefully someone else can give you a better idea. I don't know enough about the Battle of Harran to come up with explanation on how they could win.
But if you look at geography, the Crusaders taking Damascus would be a fairly big victory. First, Damascus is rich. If Antioch controls Syria (meaning Allepo, Homs, and Damascus), they have eliminated a major rival and can concentrate their forces against the Seljuks. In the south, there is only the Sinai and Egypt, which can be contained under Fatimid rule. The Syrian Desert protects the eastern flank of both the Kingdom of Jerusalem and an expanded Antioch. It is not a perfect protective boundary, but it does provide some measure of relief.
So the major rivals are now the Emirate of Mosul to the east, and the Seljuks in the north. The Seljuks are also threatened by the Byzantines and the Georgians.
So at this point, I think the Crusader states are in a much better position startegically. If they control Damascus, they have its wealth and no longer need to worry about it being in enemy hands. Antioch and Edessa can concentrate against Mosul, and the Kingdom of Jerusalem only needs to worry about Egypt.
At this early stage, the Crusader states are still fairly vulnerable. Their biggest advantage is that their Muslim rivals are still divided. But Fatimid Egypt is fairly somnolent. The big issue are the Seljuks and Mosul. Of course, simply by preventing a single power from controlling Mosul, Syria, and Egypt, you prevent the nightmare that Saladin provided when he almost wiped out the Crusader states using all those resources.
It's still possible for the Byzantines to do well against the Seljuks. John II Komenenos did very well against the Seljuks soon after this time period, and Alexius also did well before him. If they are able to ally with Antioch, the two could have some successes. Of course, it is also possible that Byzantium and Antioch would fight each other as well. Bohemond and the Norman Sicilians were always trying to do something against Constantinople. And the Byzantines are still unhappy about the Crusaders not following the vows they made.