I think it would be very interesting if they included a campaign on the third crusade, as well as in year 2, which was the most epic of all, because it faced great figures like Saladino. Another option would be a campaign that focuses on the final war between the Byzantines and the Ottomans. The end of this campaign would be the siege of Constantinople, which marks the end of the Middle Ages, and with the quality of the AoE IV walls, it would be beautiful.
These campaigns may be very good choices if we consider that Byzantines and Ottomans are in the game. From what I have seen on another thread, there may be Abbassids in the the game (that corresponds to Saracens in AOE2). So I think we might have a crusade campaign with Abbasid and European civs.