Well for one, it'd be called Pictland rather than Scotland. But other than that I am not too sure. Hell, I don't think people are completely sure exactly who the Picts even were exactly. Personally I think they were simply ancient Britons, descendants of the Caledonians who fought Rome. But I cannot think of exactly how having the Britons rather than Gaels retaining the north of the island would change history. I do have some ideas as to how to reach this outcome, however. Although it'd have to be really early as from what little I know it seems the Scots were migrating in large numbers and gaining much power even while Picts still ruled. One way to keep the Picts in charge would be to reach some PoD that involves less Viking raids on Pictland. The Vikings sapped much of the Pictish power.