If you could actively raid trade routes with war fleets then it would radically alter the need for navies in the game for coastal nations. If you could raid the trade of a country you are at war with by simply parking your navy close to their ports, then navies would become a drastically important part of the game for many nations (e.g. England, France, Spain, Portugal, many Mediterenian countries), and it would reflect why many of these nations needed such large navies in the first place, even though they weren't planning on invading people.
If trade were reworked such that there were actual "convoy fleets" moving around, that would make it even more important.
And either of these would make an important jumping off point for more development in naval play.