When you win a game, the game ends and the credits roll. You cannot continue the game after you satisfy one of the four victory conditions.
You could do what trisha suggested when starting a game by selecting only one other faction to be in the world. Make sure you don't pick Resolyn or Yithrill or any faction whose leader is "paranoid" or they might attack you for no reason. I would also pick a faction that is aligned with you so you will be more likely to have good relations with them.
Or, another alternative would be to just disable the "conquest" victory condition when starting a new game. Then you could destroy the one other faction(s) and the game would continue, since destryoing all remaining factions would not be a vicotry condition if you disable consquest victory.
That would allow you to just reclaim the wildlands. Since the final quest is part of the wildlands you would have to clear that to clear "all" the wiledlands and if you save that for last, you could consider clearing the wildlands to be your victory condition. Of course, you would still ahve the option to shortcut the game by doing the final quest before you cleared all of the wildlands, but that would be your choice.
