Seeing as mine-fields work horribly as offline protection and even system defence, I propose the following.
- X minutes after you log off, your minefields cannot be sweeped, they can only decay.
- Obviously for this to work then you no longer can put mine-fields arbitrarily everywhere. This is something that should have never been the case as it is impossible to balance something that can do so much damage by allowing it to be put anywhere. It would be like ODS firing 1200 damage nukes being able to be put anywhere in space. So therefore the mine-laying modules will be removed and replaced by am expensive tiered building. Each tier corresponds to the current mine-laying levels.
- Minefields that overlap will cancel each other out, if they are of the same strength, otherwise the strongest will win. That way it wont be that easy to colonize inside other peoples territories and putting mine-fields there. Putting minefields in other peoples territories is another thing that should not have been possible as it is highly abusable.
With this in place then you have a proper offline defence and, since they will decay, it wont last forever if you aren’t an active player.