Making rivers potentially longer can be as simple of having more chunks pre-generated but that comes at a performance cost. Making them smarter in how they conform to the terrain is a larger undertaking. I have a river generation algorithm rewrite as a side project but for now we'll be looking at incremental changes.
Very true, small steps, and nothing too bad. Lots of mod authors try to tackle big chunks at once, and end up burning themselves out. Stability first, compatibility second, then look into the fancy stuff!