This idea would require a lot of work, so I'm not expecting it in 0.24, by any stretch of the imagination, but I was recently thinking about how an ideal river generator might work. Here's the general flow of what I have in mind: You import or sculpt your terrain. A realistic terrain import would work best for this. At the top of one or more stream channels, you place a River Source object, specifying flow volume. When your river sources are in place, you activate the River Maker. Water flows from the sources, combines or diverges as it flows downhill, pools where lakes would realistically form, and makes its way to the ocean or other low point. You can let the river maker run until it reaches equilibrium (water flowing into the ocean, or off the map if there's a low point at the edge), or stop it if you don't want your low point (if it's away from the edge) to fill up. If you don't like the result, you can tweak the flow volumes and source placements and run it again. Once you're satisfied, you bake the result, which turns into a set of normal BeamNG rivers and lakes. This would not produce completely realistic waterways, which gain and lose water not just from their tributaries, but from groundwater. As someone who has studied environmental engineering, I can tell you that you do not want to go down that rabbit hole. Surface water will produce a good enough result. Running the simulation would take some processing, as would the baking process, but once the waterways are finished, the result wouldn't be any more resource intensive than any other waterways. But they'd look more realistic.
