I think I've come up with a basic method to make a bit more "freehand" looking rivers.

Rivers made with vector tools, like Inkscape (which is what I've been using because I needed tapering) tend to end up too perfect for my tastes, you can see what I mean from the examples above. I like the stylisation of the Spiro splines, though, so I'd like to just modify the rivers a bit. I tried the iwarp tool in GIMP but that is completely impractical for this scale.

I decided to use it again today, after a break for work and stuff this week again, and accidentally picked the other warp tool, which warps using a b&w filter or map as a basis and went hm.

The rivers in this:



Are too blocky and artifacty, obviously, because I did it all as a test with a 1000x1000 copy of the image, but you see what I mean. As the maps, I used two random noise maps: one desaturated Plasma render and another with a bit more smooth Solid Noise. First the solid noise, then the plasma to "rough" out the outlines on a copied layer. Then adjust the levels to get rid of the greys, and it looks pretty good.

Yeah, not going to use this exact example, but I liked it enough to ask for opinions. I'm going to edit the rivers by hand a bit still, smooth out corners and riverbanks etc, then do this process at full scale (4000 x 4000 for now should be enough I hope).