I can agree with the concept but not with the specifics of this suggestion. The idea of the seasonal enemies taking an active role against the the player(as in blockades) would be really cool, and as with other blockades would give some sort of artifact. However my problem is with the idea of getting more green badges based upon where they are attacking. As of right now green badges are one of the hardest badges to get so having one planet having the opportunity to give you three would be a bit much. So how I would change this is that I would keep the green badge at one badge per invade, no matter the location, but keep all of the cool stuff: the npc controlled planet and the ability for them to attack your planet. Maybe if they have conquered a planet, rather than getting an artifact, they leave an artifact on the surface of the planet that you would have to extract if you wanted to move it to another planet. Just my two cents
