Rivers are so limited because the stamps used to generate them are...lacking. The stamps are tiny, lack variety, and few in number. The stamp-based system is useful for some terrain features (mesas and canyons, for example), but I don't think stamps are even capable of producing proper rivers -- especially on larger maps. The only way to make correct-looking rivers and streams is via procedural generation. Simple, randomly-placed pregenerated stamps simply cannot produce the same results as procedurally generated rivers that use heightmap topology to determine start/end points, course, width, depth, etc.
As far as A21 roads go, I've had the opposite experience. On 10k maps, I've seen some pretty cool 4-way "country" tile intersections, and multiple highway entry points into cities. In my experience, only small maps under 10k suffer from really bad, inconsistent road systems.