I've had some suggestions along those lines but indeed the devil is in the details with how to implement it so that it's something you care about but something you can also ignore.
Well, I have got an Idea: You could do a command for support-ships, those you build for planetary defense, to patrol your trade lines and destroy enemy fleets trying to blockade them. If the ships you send on "patrol duty" wouldn`t be enough and a trade line would get blockaded anyway, or if you simply haven`t send any ships on "patrol duty", the planet would after some time begin asking himself why he doesn`t get any goods and would beginn sending fleets of "Patrol-Ships" along the Trade route. These are special ships, in many ways like support ships, but, maybe, depending on balancing, stronger and you can`t build them on your own, they are spawning only to clear blocked trade routes, maybe there could even be some all the time, and you can`t tell them to do anything but "patrol duty", so they can`t join fleets. After some time these "patrol fleets" would get stronger and stronger until the trade rite is freed again and then despawn again, or else you can use your own support ships.
Maybe this way we could even get pirates again, which would try to blockade trade and you would have to build support-ships (or let them spawn automatically with these defense resources) to clear the pirates away.
I`m sure this would be great and a good way to get actual freighters on trade routes and keep the game balanced. (And we could get epic fights against Pirates!)
Another Idea where escort-ships for freighters or armed freighters.