sorry for not attending this thread for such a long time. I had to divert my attention away from map making for some time- but should be back in force soon. So I just missed your tips...
The checkboard pattern is indeed a different land- a quite opressing kingdom to the south.
Thanks for all the advice, thats just the stuff I am looking for. I am an absolute beginner to Gimp and graphical design in general- so I do not really have an eye for the stuff you are pointing to. I will work on the water and smoke effects maybe something easy as the transparency can achieve something. I will also take a look at the mountains. I think the effect occured because I used different brush scales when entering them. Maybe I also added mountains in between of the creation process, thats where the different blurring effects might come from. I hope my layers are organized enough so I can adjust this without remaking the whole stuff (thats the problem with being a beginner, you never know when to add this kind of organization).
I think I will post an update of the map during the next week.
Thanks for all the advice, it is very appreciated.