Ever since legacy, I've always appreciated that the devs attempted to make darkness an obstacle that requires practical consideration. Having torches in mines for example added a layer of immersion and gave these items a purpose. However, as most of us know, the majority of the player base uses custom clients that completely negate any penalties from being in the dark at all. I think this is unfortunate. But it's obvious that there is little that can be done about this if darkness is purely a cosmetic consideration.
Therefore I think that darkness should directly affect your character's perception and even movement speed, maybe even depending on the severity of the darkness. Similar to how swamps restrict movement speed. For example, only a mild penalty outdoors on a night with a full moon, but a much more significant penalty in a cave or a mine. Perhaps even being dependant on the cave/mine level you're in, simulating the environment getting darker the 'deeper' down you go. This would obviously have a direct effect on Exploration ability as well as the effectiveness of ranged combat in dark areas, which currently has very few drawbacks and would be an interesting consideration to work around since bows require two hands. This would give much-needed utility to items that are lacking a meaningful purpose outside of being cosmetic for most players. Such as torch-posts, lantern hangers, headgear that provide light, etc.
I know that many players groan at any suggestion that inconveniences them in any way or take additional steps to do things they are already capable of doing with minimal effort. But I believe this change would be in line with the spirit of the game. Ideally, this change wouldn't be needed if custom clients couldn't simply bypass the intended effect of low visibility. But since that isn't something the devs can really control, I think a change along the lines of what I've proposed may be the next best option.
Thanks for reading.