I'm working on a 1618 scenario. Just adding the countries that are supposed to be in the game now and then I'll show a picture.
Better wait for finished 1718 one then... for the same reasons I expressed in post #2.As long as you do a 1792 one after that.
Great!
Be careful with the structure of the scenarios and especially content of the corresponding inc folder. Scenarios have additive information (except GlobalData file) for easy maintenance and only differences with immediately previous scenario in date should be recorded in the inc files. When 1618 scenario will be ready, it means 1648 scenario will have to be modified and check for changes since 1618 and not since 1520 as it is currently.
EDIT: for a 1618 scenario, I suppose it is easier to start with a 1648 scenario structure and just remove things that happened between 1618 and 1648... and as a result removed stuffs for 1618 are the one that will remain as mandatory changes in the 1648 scenario. In other words, keep a track of removed stuffs for this part in the 1618 scenario and modification of the 1648 one will be very easy.
EDIT2: be aware you can choose any date in 1618. January 1st is not mandatory. See here.