I would take the 1.0 storyline and add it too the ARR onwards world all be it the involved zones aged back 5 years.

It wasn't all that long of a game and we can skip over the stuff like 1.0 crafting etc.
Just the 1.0 MSQ as a 'tutorial', Its notable side quests can be added as part of it etc.
That section of the game would need to be partitioned off most likely unless SE added a phasing system that lets players at different world 'age' stages be on the same map and see each other but also see the world matching the time period they are in.