I’d like to propose an idea about planetary escalation.
Two variants:
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An RTS (real-time strategy) session against NPCs. Gather resources, build a base and army, destroy the enemies.
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A Tower Defense session. Defend your planetary base.
If victorious, for example, a bonus to the planet’s production could be granted. If defeated, a production debuff.
The more efficient the system built, the higher the chance of triggering an escalation, and/or the less downtime over a certain period, the higher the chance of getting an escalation.