With Ned at the Wall, there'd be no immediate reason for Robb to declare open rebellion and march towards King's Landing. He would rule at Winterfell, grow his forces, tighten his grip on the North, and eventually make plans to divide the realm, though it would've been a much more considered move as opposed to a dash south. Then think of all the trouble that Robb would've been spared. He wouldn't have died, for starters, because the Red Wedding wouldn't have taken place. He wouldn't have been betrothed to a Frey daughter, and would never have met Talisa, and would never have got her pregnant. Robb would've have a fresh slate, and Catelyn would still be alive.