The Once Human team is talking seasons and scenarios, along with expectations for new content rolling out. Scenarios are set within seasons, and have their own content, rulesets, and more possibilities.
Once Human launched with two scenarios - Manibus (PvE) and Evolution’s Call (PvP). September and October will each bring new scenarios. Next month, you can “face confrontations among factions” in "Primverse's Clash" and in October, the scenario will “introduce you to an all-new map and challenge you to survive extreme environments”. There will be variation in upcoming scenarios with a limitless PvP one in December.
While the goal is to release four new scenarios per year, future content will also include one regular scenario that is not tied to a particular season that will be available at all times.
So, every season gets split into different phases that are determined by the features and lengths of the scenarios. Each scenario has its own rules, some might need you to survive the environments, fight monsters, maybe explore and secure some Deviations. Maybe you need to form alliances with others and work together to secure supplies, including in competition with other alliances or solo players.
Special rulesets may also apply, like additional obstacles that make it more challenging, like harsher weather. Some scenarios may also add a completely new map we haven't seen before. Some may even feature different varieties of combinations that form the challenges within. scenarios will also potentially scale up or down, so the max number of players may change.
In-game content will unlock gradually during these seasons. Once you unlock all of the scenario content, and the season hits a settlement phase, you can pick another scenario. If you don't want to sign up for another scenario, you can just hang out on the server surviving and doing your thing until the settlement phase ends and the next phase begins.
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There's more info on how progress will count and what will persist between seasons and scenarios, and even how you can take items from your Eternaland Depot into scenarios.
These phases are four weeks, and as things move forward, seasons and scenarios working this way is the long-term plan.