Playing as England I got a "free" idea - Quest for the new world. The year is now 1495 and I've discovered almost the entire world. No other Western country is exploring and thus no other country than England has colonies. It seems to me that it's way too easy to explore. All you need is a small navy, an explorer and the odd diplomat to gain military access to a large number of new world countries (+25 relation is quite enough). It could be that my country is too strong compared to the rest of Europe, but making it much harder to sign military access treaties with new world countries, would surely slow down the exploration pace.