Small suggestion that I think would make the game a lot more immersive;

When you connect strategic/luxury resources to a city, or set a trade route, I would like to be able to visually see a movement of goods on the map (similar to how roads become visible).

Now trades routes are practically invisible, whereas I'd like to see it actually having an impact on the map. It might be just a cosmetic changes (seeing carts / ships move across the map) or also functional, being able to pillage these routes with army units (which could introduce a need to protect these routes, which might also make sense historically), or it could be coupled with some districts that can be built along these routes.

Since some of the underlying mechanics are already available in the game (calculating roads, trade route routes) hopefully this might actually be a feasible addition.

Hope this sparks some interest!