In many worlds Duncan turned right at the crossroads near re-entering Ferelden, in many worlds he visited either the mages, the Dalish, the Dwarfs, the Warriors or the alienage elves. In one world though - this world to be precise he was diverted long enough to be denied entry into Ferelden by a few more fanatical devotees of Loghain. This caused quite a few changes to the 6 would be destiny's of the Ninth Dragon Age.
At the Mages tower a young Apprentice passed her harrowing only she was never called to meet a grey warden, with no grey warden around Jowen's blood magic was never suspected as the Templars weren't on high alert. The young mage of the cast from the Amell line took to desperation to leave the tower and sought out blood magic, her friend Jowan, recognising the signs taught her in secret, by testing blood magic spells on each other in secret they grew more power and carefully planned their escape.
As Duncan never passed into Ozarmar he never caused a dozen new Dwarfs to come up from darktown to see the Grey Warden meaning one smart mouth Dwarf never insulted Oghren and thus never was killed forcing Oghren to make another trip into the deep roads to find his wife. Also in darktown the time that it would take Rica to find a husband was not put into question as with no Grey Warden coming, the two sisters boss didn't need anyone to enter the proving under false pretences to improve his earnings.
In the Brecillian forests a group of humans didn't leave town with a Grey Warden coming so they could avoid possible conscription and thus didn't end up getting killed by the elves in the forest, nor did they find the cursed eluviania in the underground city of the elven and thus the elven tribe never needed to move and the scouts never even suspected the entrance.
In the elven Alienage in Denerim a group of rich boys didn't get kicked out the palace as a Grey Warden was due to arrive and the Arl didn't want his son to be present and disrespect someone who could conscript him. Thus the party of 'men' got to rat-arsed to even leave the estate and thus the wedding of the elven to keep the inbred bloodlines going, went off without a hitch.
Meanwhile the Darkspawn continued to advance but with no grey wardens at the frontlines, the King Callian stayed back and this allowed Loghain to actually plan a tactic to destroy the darkspawn safely from a distance, with catapults and archers, no men were lost in the siege because no one had to charge stupidly with the Grey Wardens in the front lines against the enemy.
Because Loghain was in charge and no grey wardens entered the lands, Howe saw fit NOT to attack one of his closest friends and the Warriors were safe.
Of course none of this stopped the fact that the archdemon was still on the rise, rebuilding its army - which would take a lot longer seeing as it got no new recruits at Ostagar.