In short - it'll be a much slower-paced, slower-progression, slow-building mode where you build a permanent colony over time and you deal with threats as they arise, controlled by a "director" routine which decides what happens and when. This mode will start in a simple form, but there's a ton of room for it to expand in all directions. Weather / seasons / one-off super enemies / disease, etc etc.... all things that can be added over time.
We don't see us making many assets specifically for freeplay; instead we'll take the ones we make for the formicarium and repurpose them.
Freeplay, by its nature, is a sandbox mode and doesn't have an endgame. Instead, it's all about running your own colony, in the long term.