An option might be an alt-Hadrian (to go waaaay back).
Rather than build the wall, he insists on pushing the frontier north, and settles Romans and any populations he wants to relocate to "Northern Britannia". This includes a large number of Roman soldiers, who marry locally.
Otherwise, not much else changes outside of the British Isles, the Anglo Saxons still come along and cause an alt-Heptarcy, and Europe goes as Europe does, but Scotland is now more populous.
This leads to an alt-Scotland taking OTL Northumbrias role - extending from the North down to the Humber - based around the Firth of Forth.
Whilst is isn't going on foreign adventures, and isn't strong enough to dominate the island yet, it is certainly well placed, and with the Romans urbanisation efforts maintained - largely inured to Viking Raids. England however, is not. In combination with a discovery of Vinland by the Vikings, and a successful settlement, Scotland now forms the heart of the trade network, unlike the Norse, but strong enough to ensure they don't invade. Eventually they follow into Vinland, and start to settle North America earlier than the English IOTL.
Now admittedly, this might not qualify as Scotland as it was part-Roman, but I figured you want as much of an urban boost as early as possible, to make up for the unfortunate lack of high-yield agricultural land.
After that - who says this Scotland couldn't fortify at the Humber, with a Greater Wessex in the South, and a uniting Ireland in the west. A three-way tie on the islands, with Scotland and Ireland leading when it comes to the New World, and G.Wessex dominating the Channel trade. (The idea of a Scottish N.America and an Irish Caribbean is amusing to me).