As the ancient writers seen to have had some knowledge of Iceland (Thule), I've always been curious about the establishment of a Greek or Roman settlement there, long before the Celts or the Norse arrive. All you need is one ship of colonists and then what? The cultural development in such isolation would be fascinating. Would they maintain some semblance of their language or culture, or eventually find themselves entirely assimilated into arriving Celtic or Germanic culture?
Of course, there's not much reason for Greeks to settle such a far-flung, empty, cold land.
The Sub-Saharan African coast is another interesting direction.