As I have said in another part of this forum, Devs might already be thinking about something like this, but still:
A kind of Mega-Event: The Chosen One's quest to save Arroyo.
This megaevent would start the day in which the Chosen leaves Arroyo (July 25, 08:24, 2241) and would include Mini-Events to let the players help the Chosen, maybe with the most important Fallout 2 quests (help a particular New Reno family, decide who gains power over Redding, help the people in Vault 15...) or maybe try to make him fail. This way, players would write Fallout 2 history.
Of course, the oil rig Enclave base is to be destroyed in the end, Arroyo saved, etc...
Maybe after that there could be changes in the Enclave NPC faction? Probably in San Fracisco too as the Chosen gets help there to reach the oil rig.