I think the bigger question is where will new players start on the storyline? Will they have to go through all of 2.0's story quests before being able to start anything in Ishgard?
If they DO have to go through all the MSQ missions we've already done it's going to take a LONG time before someone who bought the game for 3.0 gets to what they wanted to play. Add to that, all the lv50 quests you get after Praetorium are going to have to give EXP as the levelcap won't be 50 any longer.
If they DON'T have to going through the MSQ missions, then how are they going to gain access to current content as the majority of that is walled behind the 2.0 MSQ progression.