I'd like to see the movement of units over water being limited. perhaps it should not be possible for countries in one region to colonize on another until they reach a certain tech level - to signify the ability of that nation to build ships large enough to traverse vast oceans. I think this could be done by either making it so that the region, or continent a civ starts off in is the only one it can colonize on until it reaches a tech level (or some other trigger). Alternatively, there could "simply" be a limit to the number of sea tiles a unit can travel through that is based on tech level.
The situation in game at the moment: I started a new random game. I was looking forward to sending the picts to oblivion during a war with them. I took all their mainland provinces but didn't get 100% warscore - because the picts had become the settlers of the vast wasteland over in northern canada! Another thing that happened was that some random civ on mainland spain had gone to war with some place in north america and taken a large chunk of land by about turn 34. seems like the world should be a bit more isolated until a later tech level at least
and none of this is to say that the AI couldn't use a tweak as to when and where it choose to put down colonies