The inhospitable icy wastes of Frostpunk don't look so bad if you've picked Serenity, one of two routes to take through new free Endless Mode DLC. According to 11 Bit's video dev diary (which you can find below along with a launch trailer), Endless Mode took longer to develop because they got a little carried away on the new content front. There are four different maps to pick from, determining how free you are to expand your icepocalyptic city outwards, and players can pick between two styles of play as they build without the threat of a looming, story-ending disaster.
Frostpunk's campaigns have always been timed affairs - there's a big storm or other disaster on the horizon, and your goal is to fortify well enough to hunker down when it hits. Endless Mode removes that limit, letting you build your city to personal preference and at your own pace, so long as you can keep the resources flowing. Mega-storms are represented in the form of regular resets to the world map, revealing new sites to explore and gather precious materials from, while burying others forever. There's also relics to be found out the snow, with new lore attached.
These relics belong in a museum, and that's where the new Archives building comes in. Once built, Endless Mode players will have an extrinsic goal to work towards, outside of building the warmest, safest city. That goal is somewhat easier in Serenity mode, but in Endurance mode, you'll have fewer resources and will find your city occasionally hammered by random events. It's probably best to make sure you've got some redundancies, just in case all your automatons freeze over. It seems like there's new events to deal with both inside your city, and out in expeditions.
Further diverging from the harsh pragmatism of the main game, Endless Mode also introduces a bunch of decorative items to adorn your city with. Best to get it nice and photogenic, because 11 Bit have plans to release an expanded photo mode before the end of the year. Right now they're playing their cards close to their chest, but the studio tease more ideas and updates coming in the new year. Given that they've still got more DLC in the pipeline for their previous game - This War Of Mine - it feels a safe bet that Frostpunk will be evolving for a while yet.