I'd like a hardcoe mode that is more... I guess, hardcoe. Maybe a degree of hardcoe mode instead of just a single setting.
I personally like mods that consider eating/drinking/sleeping in a reasonable manner, where food will decay.
Extreme heat/cold being dangerous
No map-based fast travel, but a rather open-ended paid fast travel system (like paying a vertibird to drop you off at any POI on your map, but being responsible for returning on your own)
An advanced injury/repair system.
But I know some people don't like all that added trouble, so maybe a difficulty option should adjust more world elements than "do X% more/less damage, take X% more/less damage"