Actaully have them centralize into a single or larger state as opposed to being a miltitude of city states. This made them easy prey for the Dai Viet who were eventually able to conquer them slowly one by one. I suppose a Champa state would cover OTL Southern Vietnam while the Dai Viet would remain in the north and possible expand west into Loas instead of the South.
A Champa state could resemble something along the lines of Thailand where a Thao ethnic group is creates by incorprating many ethnic groups. If it becomes a muslim state this is a strong possibility.