USA: Not many choices. Kill as many Al Qaeda and ISIS troops as possible. Once it looks like Assad can survive, just walk away.
Israel: Tell Assad that Israel is willing to accept another 17 years of Cold Peace with Assad as the Syrian leader, but tell Assad he MUST begin to dial back his support for Hezbollah.
Both plans are probably dead on arrival, but the alternative is multiple armed groups, occupying multiple areas inside Syria, possibly for multiple decades into the future.
then what should happen is to lay absolute waste to Syria, if it is native to Syria, eradicate it.
harsh but effective.