Emperor Karl (Charles) von Habsburg was the last imperial leader of Austria-Hungary. Karl, his uncle Archduke Franz Ferdinand, and various imperial reforms sought to save the Austrian-Hungarian Empire by remaking it into a federal empire known as the United States of Greater Austria which would allow minority protections. This, the reformers hoped, would strengthen the empire by pleasing all but the most harden nationalist dissenters while keeping the empire intact. However, the plan failed to get out of the empire's bureaucratic boardrooms during World War I and the empire died when various minorities violently split at the war's end.
The idea a united country comprised of different, sometimes but not always hostile groups with local autonomy is still sometimes proposed for national unity and it still works. Ethiopia, Bosnia, Nigeria, and Iraq have federal system which allow local minorities great protections and local freedoms. Granted, some multinational countries in Africa and Asia are splitting themselves apart but this is usually because one ethnonational group has set up the country as a nation-state at the expense of others.
For Syria to work ethnosectarian autonomies must be established like Archduke Ferdinand and Emperor Karl favored for Austria-Hungary. Each mini-state needs to have input into the functioning of the federal system (bottom-up). The top-down system worked as long there was little to no sectarian tension. However, the civil war made this tension a reality and it is impossible to simply wave away the tension for now.
There are two foreseeable solutions. The first is a sort of United States of Syria which would be similar to the old French mandate system though a Kurdish state would be a necessity to ensure peace in the now de facto independent northeast. A rotational, and weak, presidency would give each ethnic group their buy-in and say in federal affairs.
The second solution is a unified Sunni ministate, a unified Christian-Alawite-Shia-Druze ministate, and a Kurdish mini-state. Perhaps Lebanon could be included thus restablishing a united Syria which was broken up by the French. Though I admit this would be difficult and therefore unlikely due to Lebanon's fragile balance and Lebanon's Sunni population would be strongly opposed to this idea. A joint-federal government like that of Bosnia with a Serb ministate and a Croat-Muslim ministate proves this could work. A writer on Pakistan's Ministry of Defence website even made the below map to show this idea would look like.
A top-down governed Syria is dead. The only way to reestablish one is with the horrible option of Sunni mass killings of minorities which would cause an even greater refugee crisis. Only mixed government with local autonomy can keep the empire of Syria together. It must learn from Austria-Hungary's lesson of being too late to change.