[Scarmiglione]

Silverelf: You can switch around the nations on a scenario map by editing the .map file of the map in your dominions 3 map folder. lets say you want to change Oceania to R'lyeh. Make a copy of the dawn.map and dawn.tga and rename them into something like Rdawn.map and Rdawn.tga (this is to avoid overwriting the original dawn of dominions.)

Then, open the rdawn.map in notepad and look for #dom2title. change "dawn of dominions to somnething like "dawn of R'lyeh)

Next look for #imagefile. Change dawn.tga to rdawn.tga (or whatever you renamed the .tga).

Next, scroll down to #allowedplayer. Look for EA oceania, which is nation 26. Change that to EA R'lyeh, which is nation 22. (the nation numbers fo all nations can be found in the mod manual)

Finally, find the command #specstart 26 49. remember, 26 is EA oceania, so change that to 22 for r'lyeh. the 49 is the starting province, leave that as is. You may want to change #landname 49 to "R'lyeh" for flavor purposes.

Then start a new game on the map "Dawn of R'lyeh" and it should work fine. You can replace any other nations, not just R'lyeh. Just refer to the mod manual and switch numbers where appropriate. You can even add nations from other eras, if you make a simple mod to put them in EA (the mod would use the commands
#modname "(nationname) in EA"
#selectnation X(X = nation number)
#era 1
#end.
Then enable that mod, and do the above steps using whatever nation you picked.

I think I've remembered all the steps, but post if it doesn't work or my description was confusing, I will clarify, or one of the more knowledgeable posters here will probably correct any areas I was wrong about.