Oh, one last idea, after talking with Annikat about it - if you threw in like a fate card maybe that essentially retriggered the last wave or sent some % of the upcoming raid's power, it would give people a reason to keep some defenses on hand at all times, along with a gold upkeep to make gold production more necessary. Obviously, using the last wave instead of the next wave for numbers gives the player a better chance of being able to predict and prepare for the random event, but either option sounds decent