First, this is a fantastic tool, thanks!
I was thinking about the view
with
waterway&nametags. grouped by topology & name
and I think it’s a shame rivers are divided because they have different name=* tags when they cross borders. Would it be possible to see if two connected rivers have same name:xx=*, in any languages? If any language matches, it’s the same river. In my experience this would be a perfectly fine approximation, it wouldn’t use relations, and a lot of rivers would be correctly grouped already.