Something that I was wondering about was the implementation of "goods quality".
This is almost definitely impossible to implement for the base game, as it would be a ton of work, but maybe it could be a DLC idea.
The obvious case for this is firearms/cannons; as you advance through the ages and have better production methods, those should increase the goods quality of the firearms/cannons you produce, making them more competitive on the market, and making any troops that use those weapons better.
This could apply to all goods, though, and could lead to specialization among different countries, and would make technological progress more impactful (could also fix the issue with Asian vs European spices, as Asian spices might be considered higher quality than European ones, same with silk and porcelain).
Quality doesn't have to revolve around advances only, because of Project Caesar's new cultural mechanics. Maybe certain cultural products (glass, fine cloth, lacquerware, porcelain) can increase in quality depending on the cultural strength of the country in which it is being created.
Just throwing out some ideas, I think this would be a really cool system that would add so much immersion and depth and synergize well with a lot of systems you guys already have developed.