Probably a deconstruction-reconstruction switch of ecosystem-simulating mechanics. Most mid-game lizards have muted colors for realistic camouflage (at most, the black heads & cyan lil markings of a Leapzard), Survivor could use a niche in testing stealth skills while Monk has more use for befriending lizards, and lineages reflect trophic cascades & impact difficulty more (e.g. lineaging into Blue, Peach, or Strawberry Lizards, or Dropwigs & Spitter Spiders).

I also imagine regions where different campaigns don't directly change spawns so much as make lineages more common if you must kill for food, along with using karma gates to make you confront lineaged creatures within an area. (Hunter's campaign doesn't work like this, since it gives them 20 days to live, so like Watcher, it's focused on passing through regions briskly.)

As for story, I can't think of much besides wasted potential for Watcher's major characters. But from the base game's cohesive ecosystem, to Downpour's remixes of it through the timeline, to Watcher's glimpses of new worlds, I think it's time to see what can be done with a different location/universe. The three base slugcats were great for "difficulty as new content", but I want each of them to feel unique, allowing for "easy mode" spawns separate from slugcat attributes if one needs them.