I've been experimenting with creating large maps with simutrans-experimental. I've been trying to get rivers which are useful for trade early on.
The first noticable problem is that cities don't particularly cluster near rivers. They should (historically, they really did). This would require tweaking the city generator though.
The map generator doesn't create many rivers by default; not nearly enough for a satisfying large map. However, you can turn the number up. This also mitigates the problem with the city locations, as they're more likely to be near a river if there are more rivers.
The real problem, however, is that almost none of the initial rivers created are navigable -- on a large map. I think the river types are chosen by starting with streams and enlarging when two rivers meet -- I'm just guessing here, but it matches the results I see. The trouble is, on a large map, the rivers rarely meet. If you turn the river count and length up high (I maxxed both out), they will meet often enough to make *small rivers* (rather than just streams), but you will still get very few wide rivers on the map.
Accordingly, in order to make rivers useful for trade on a large map, small rivers should be navigable. (Thematically, they were navigable by smaller canal boats, though not by ones with large drafts -- weight limits should deal with this, with a smaller weight limit on small rivers than large rivers). I think this is an easy change which would make the early game a lot more fun. Streams can remain as the "just an obstruction" type of river.
EDIT:
To be clear on the problem, this is related to the use of large maps, because rivers are one tile wide, not a width proportional to the number of tiles in the map. With smaller (512x512) maps the rivers meet appropriately and create enough "Rivers" and "Wide Rivers" for trade. But with large maps, such as are used with Experimental, there are very few "Rivers" and "Wide Rivers". I think the simplest and best solution is simply to make "Small Rivers" navigable in the experimental version of pak128-britain (not in the standard version).