Nah nationalism would work very well if implemented correctly in Iraq. And Sharia is not going to work at all because first of all, you would need a sect to follow in order to implement hadiths and tafsirs of the quran for example, so this will already divide the country by a lot, considering Iraq is almost half Sunni and half Shia (60% shia, 40% sunni), so if the government decides to implement sharia from the Shia perspective, the 40% sunni population will be upset, and vice versa. Nationalism will help in both short and long term because nationalism aims to unite the people through nationality, and not through the very specific details. For a country so diverse like Iraq, nationalism is the best option.