In my point of view, Though US has rejected meeting Russia in Geneva , they should meet again and should think of dividing Syria into two, one in favor of Assad and another to form their own government, the way they prefer, it could be democratic or whatever. Like Qatar was once controlled by Bahrain, Pakistan was India.

Or if they don't agree, let them fight each other until the end but as per my analysis Foreign intervention is more dangerous than allowing them to fight each other.

Or if they really like to intervene they should destroy both Assad regime and the rebel forces and the civilians form their government and let them build Syria from scratch.