Hey Obsidian! Was a huge fan of Grounded when it came out and loved all the creative mechanics introduced, but one thing I always felt could be fitting would be a system for Rain, and how dangerous the typically uninteresting weather pattern could be on a miniaturized scale.
Design wise it could be rather simplistic, just a large shadow appearing across the map with the telltale bombardment sound applied, then a sudden splash that affects different sized insects in different ways, and also drop down two to three water droplets to drink from! Force players to need to seek shelter inside their built homes, or in caves or beneath objects, to avoid taking splash damage (Hah, pun) from the downpour. And like a few other things in Grounded, it doesn't have to be a one to one scale of ACTUAL rainfall, just enough to startle the player and make them need to rethink what they're doing until it's over in a minute or two
Could say harm/kill most smaller insects, causing them to scatter. And only have a minor effect on medium/large insects, then colossal creatures would be completely unaffected. And imagining how hectic it could make the player if they're strayed too far from their home base to now seek out a safe spot that isn't infested with other insects sounds like it would be an awesome challenge to include, with an easy way of getting water early game in case a stockpile was running low.