First attempt at World Map for my campaign: "A Ballad of Bones and Scales"

[Meshon]

I was just going to add, if that's a bit too rough, you could use the fuzzy selection tool again and play around with its settings. In this step I selected the empty area, inverted the selection, and then filled it again.
Screen Shot 2015-04-16 at 11.21.12 AM.png
For sure ask me if you have any questions and, like I say, someone who knows their GIMP may well come along with a simpler/better method.

cheers,
Meshon

Edit: I should mention, this is not going to give you realistic coastlines, but it will break up the smooth and give you a bit more to work with.

[johnvanvliet]

there is a old and still working gimp script
fractalize_path.scm
http://registry.gimp.org/node/17717

see the demo image
http://registry.gimp.org/files/frac_demo1.jpg

[Sushipopsicle]

I'll try this out right now! I haven't worked with paths before, should be educational if nothing else.

[Sushipopsicle]

A Ballad of Bones and Scales (Jagged).compressed.pdf

Here you go! I tried your method, and I eventually realized I needed to apply it multiple times to get a reasonable effect, but it's definitely a step in the right direction.

A few of the cities ended up underwater, so I had to move them a bit, and had to recreate the oceans for shading purposes, but it's a lot better than starting from scratch.

The only sad news is that those little islands in the gulf/bay off the Lunar Peninsula got erased and I didn't notice until it was too late. But they weren't important, so I'll survive.

Edit:
Just realized my rivers are messed up now. I'll fix that on my next go-around.

[Meshon]

Right, sorry, I didn't talk about resolution and brush size at all (mainly because I didn't think of that!) which would definitely affect how finely the process would affect your design. Sorry about the islands. Maybe in the distant past some well-meaning but slightly inept god tried to fix things up and ended up sinking them. Now there is an undersea ruin there known as The Mistake...

Keep at it, this is going well!

cheers,
Meshon