http://www.europa-universalis.com/forum/
IMHO, Europa Universalis 2 deserves to be checked out by any self-respecting scenario maker.
The game ships with 220+ nations that can coexist in real-time on a detailed world map. The diplomatic system is realistic and has excellent handling for aggressive countries. The abstracted economy and military concerns are excellent with inflation and attrition respectively playing very important parts. Finally, flavour is added with a detailed event system. Oh, and it has Social Engineering.
Pros:
-there are roughly 2 (or 4?) billion event slots
-have I mentioned the 220+ concurrent nations?
-the community is very active and is currently working on a set of new events en masse
Cons:
-one needs to draw a coat of arms and an animated flag to add a new country
-there is a limited number of slots for new countries
-incompatible scenarios require multiple installations or a lot of copying/pasting
-only dates between 1419-1820 display correctly
-un-editable map