What is the Rimworld Ideology expansion?
The Ideology expansion is focused around the idea of allowing players to roleplay more. In this expansion, players will be able to create a belief system and their colony will act it out. This can be as varied as tree-worshipping cannibal to dance party techno ravers, with all sorts of things in between as you can mix and match core elements of the belief system, and customize a large variety of elements including individual precepts, rituals, and special social roles.
Beyond that, there is a new quest category where you can hunt for ancient artifacts in an Indiana Jones rogue-like challenge as your colonists break into ancient structures, facing unknown dangers for a chance at that ancient treasure. There are new special trees that are symbiotically linked with dryads who can be controlled and grown in different castes. A lot more was hinted at in the announcement, but we'll need to wait for all the details.