This is a suggestion to increase the challenge of the tiers just a bit. After changing the world tier, you have to wait until the next day to change it again.

Additional idea, but this only works along with the first idea: Only 1 Tier change can be made at a time. Example: 1 can change to 2, but 1 can't directly change to 3. To change multiple tiers it takes multiple days. This makes the highest tiers the riskiest because you can't just turn off invaders over a single day, and then return directly to a high tier the day after that.