Map Editor/Generator
Hello All,
I am interested in creating a custom map based off a real map. Is this possible? 

I'm afraid not.
You'll always have mountains all around your map, to block pathing on the edges.
You'll always have a river going from an edge to ano other edge, you can have more streams, but not another big river, and you can't have zero river.
You can decide how hilly/mountaineous the map will be, but not where the hills/mountains will be.
You can decide how many lakes you'll have and how big they'll be, but not where their centers will be, how they'll intersect and how hills/mountains will create odd shores or islands.
You can change the ground texture, but it applies to the whole map, you can't decide to have 2 different textures used in different sides of a river for example.

Basically you can adjust parameters that change how the maps will be generated, but you can't make a precise hand-painted map.
I could actually be ok with those parameters. Is there someway i can ensure the main river will flow at a certain location? And can i add lakes/remove lakes?
I am after remaking this

Attached Files Thumbnail(s)
As far as I'm aware, you have no control over the river's path, or where the lakes will be. And you can't modify a map after it's been generated. Only some serious hacking could do that, and I have no idea if anyone has tried.

It's possible that a map close to what you're looking for exists (except for the lakes right on the edges, that's impossible), but you'd basically need to check them one by one. Your best bet would be a "flat plains" kind of map as they tend to not have too much water, and almost no hills to disrupt your building.

Big edit as I just remembered there's a "Terraform tool" mod around, so you technically could cover the lakes that you don't want on your map, and create that White Knights' Castle island/moat area with that. The downside is that it raises squares by a set value, so you can't have nice slopes and often end up with pretty ugly edges around the hills you create or moats you dig. And when you fill up a lake, the lake bottom texture gets flattened and looks pretty ugly, forcing some creative building and using lots of ghost roads to create better looks.

Once again with that tool you could get something that looks similar to what you're looking for, starting from a Flat Plains map that has a suitable river and working on filling lakes as needed. The only thing that will remain totally impossible is the lake right on the edge. That'd simply break the map.

Terraform tool thing is here: