For the invasion tactics which remove tiles on the planet (reduces the class of the planet), which I will certainly never use because of this (I don't like to reduce planet quality), I should prefer a penalty related to the use of the tile for a specific duration for each tile.

Ex:

Possible penalty:

3 to 5 tiles damaged for a duration of 20 to 50 turns.
If there is an improvement on the tile, the improvement is destroyed.

Penalty after a succesfull invasion:

1 tile damaged for 25 turns.
1 tile damaged for 32 turns.
1 tile damaged for 27 turns.
1 tile damaged for 42 turns.
2 improvements destroyed.

The remaining number of turns for the penalties are displayed on the damaged tiles.

The player cannot build an improvement on a damaged tile.


Much more entertaining for me, rather than to definitively reduce the quality of a planet (and also great for modding).