Rebels do eventually 'turn back' to the fold. After awhile there is a chance that the rebels will regoin any faction that held the province they are in. So areas that start off as rebels can only 'go back' to rebels... but places like Spain or France tend to have regions which could go back to the Spainish or Almohads, English or French or Germans or Italians (toulese) etc

I would like to see the AI in general be more aggresive and oppourtunistic. I would also have liked to seen rebels treated as factions particularly in a civilwar... but it would have been nice to see to rebel provinces join into a shakey alliance then become a faction.