Maybe make the reward for a raid equivalent to the average or the total sum (to that point in the day) of the ports daily income for the port that gets raided. If that cant be done, reward the raiders with a taste of whatever resource makes the money in the port like carta tar or copper ingots etc, and nullify that days clan income for the port that gets hit. Presumably you would hit valuable ports of your enemy. This would force a defense of some sort as repeated raids would drain that clans ability to pay for other ports upkeep over time if the raids go unchecked.
My personal suggestion would be to revive the flags and put a 15-30 minute lifespan on them to create a max range and create strategic value for otherwise useless ports in the vincity of the ultimate raid target. This would force a nation to A: expand a zone around valuable ports to create a buffer zone and B: force a nation to take a port close enough to plant the flag successfully. Make the cost of the flag something absurd like 300-500 pvp marks to A:make it a focused group effort B: make it easier and less controversial to form the group of people to carry it out (contributors) and C: make troll flags counter-productive and unsustainable to repeat and D : give pvp marks another use besides a trade chip and above all make them a means to further CONTENT, meaning (presumably) people will be more willing to risk assets via pvp if there is something to be gained in the end (light at the end of the tunnel). Make it like a tow but for the whole nation, one per day max-whoever can pay for it first gets the right to pull the flag.
I dont know anything about coding but if its possible (and if it didnt take too much effort) create a unique battle environment/setting using the regular port battle map for a specific port, and using the flag itself like an item that spawns the instance when you use it near the port(within a close distance). Since there are limited ports on the map that make millions of gold, just make a small list you can choose from when you purchase the flag in a national port so if they were to code in special battle characteristics it would only have to be done for 5 or 6 ports. If successful, maybe more later. It would be a tangible form of economic warfare.
Honestly i just wanna see raids or some other mechanism that coerces or creates new avenues to ow pvp. We really do need something. Obviously there will be issues with this suggestion i understand this and i am by no means making demands. I havent contributed much to the development just thought id try.