Ok, here are some tips/comments I have found for this type of map:
Put a noise layer over your land and play with the mode. I ussually use multiply or perhaps hard light This will give you some more and takes basically little to no time color variation to your grass lands.
Good thought on the swamps. The background probably needs to be a bit darker... perhaps make a new transparent layer with a slightly darker green than the grass layer and then put the brush on top? Not sure as I have not tried to do any swamps yet.... With the grass coloration underneath, it looks to much like grassy brushland or heath or something like that instead of swamps. But, it was a very good thought and attempt!
You may want to try RobA's tutorial here: Making Mountains in GIMP for a start on something that might be more mountainous looking. Also, there are a few variations of this technique in action here: Playing with mountains to make it a bit more satellite like in apperance.
You may want to a 2-3 px G Blur to the mountains and forests. The spread just looks odd..
Nice font. Very "regal" looking.
Over all. Very good work.