Having played using the Primitive Survival mod I have to suggest having traps in a later update. Makes early survival a little bit more bearable and encourages players to think a little bit more (should I eat these berries now, or should I use them as bait?)
An addon to the above would be some non-lethal traps in order to catch small game for domestication efforts.
A structural integrity system like in Valheim, if done right, would be fun too. Build your house right or else it'll collapse with the first stiff breeze. Be sure to build some supports when you're mining, be it in an actual mine or in a cave, or else you might find a whole mountain (perhaps literally) of rock and dirt falling on top of your head.
Speaking of caves, cave mushrooms would look nice, give you another source of food, and can even give a small level of light in the area. And underground ponds and lakes filled with blind fish would make for an interesting fishing spot.
I don't know how it would be implemented in a way that wouldn't ruin the fun of the game, but cave hazards like gas and coal dust would be interesting to see. The only times I've ever seen that in a game was the Glenn's Gases mod for Minecraft and a MUD called CLOK (shut down two years ago, but was rebooted as COGG).
Some primitive steam power would be an interesting mechanic. Maybe not quite steampunk (although with the locusts in game it wouldn't be that out of place. Or would that be more clockpunk?), but something as an upgrade to windmills/watermills late-game.
Early gunpowder weapons would make for a fun late-game weapon.