Milestone unlocks

A milestone is a predicate over aggregate state (population at least 1,000, output of an item, cumulative revenue, a tier of residents) paired with a set of unlocks. State owned: the ordered list of milestones and which are reached. Update rule: periodically test the next milestones; on success, permanently unlock their content and notify the player. Unlike research, the player spends nothing directly; progress is a side effect of growth. Canonical forms: Cities: Skylines population milestones that unlock services, zoning types, policies, and map tiles; Anno resident tiers whose population thresholds unlock new buildings. Satisfactory names its HUB unlocks milestones, but they are paid for with item deliveries, which places them closer to a tech tree.

Growth is rewarded with options. The player expands toward the next threshold, gets a burst of new tools, and learns systems in the order the designer chose rather than all at once.

• threshold variable: population, output, money, or satisfaction
• spacing of thresholds, linear or geometric
• which content is gated and in what order
• whether milestones can be lost if the variable falls back
• option to disable gating for sandbox play

• Gating essential services, such as sewage or healthcare, behind milestones forces players to build broken cities before they can fix them.
• Thresholds hit too quickly flood the player with unexplained tools.
• Thresholds placed far apart produce long stretches of growing without new decisions.

• Players inflate the gated variable artificially, such as dense cheap housing to reach a population tier, then rebuild.
• Milestone order shapes city or factory layout, since early buildings lock in space before later ones arrive.
• As a tutorial pacing tool it teaches systems in sequence; experienced players disable it.

• Can milestones be selected adaptively from what the player has not yet engaged with, rather than fixed by the designer?
• Does gating by behavior (built a working transit line) teach better than gating by aggregate size?