I am just about to finish the final quest However from what I understand once i finish the main quest the game will not let me return to the world map. With this in mind should i wait to finish until the new expanision DLC is out and uoloaded? Or will the new expanision change the fact that you are not allow to go back to the main map?

The game saves a special savegame before the point of no return so you can reload after you complete the game to play any DLC. They have not said much about the upcoming DLC yet, but I would not expect that any of the 3 DLC's will allow you to continue playing after the end of the game.