I wanted to take a stab at them myself, but I'm worried I might make it too needlessly complex. To give you an idea of what I mean, I have an old doc with 7 different light levels and 8 sound levels. I do still think these could be useful, depending on how they're implemented.

Here's a list of the light levels I have. The general idea at the time was that creatures would be categorized as diurnal, nocturnal, or subterranean, which would determine their optimal light levels (so instead of Sunlight Sensitivity, drow would just be classed as subterranean, for example). Diurnal sees normally in bright and lambent light, nocturnally in lambent and dim light, and subterranean in dim light and darkness. Going one step up or down from your optimal levels is treated as lightly obscured, and going two steps or more up or down is heavily obscured. Also, a light source is considered one level brighter than the light it produces, so looking directly into a light source can blind you. Darkvision allows you to see normally one step lower than normal, and brightvision allows you to see one step higher than normal. (And Angel's Sight can also function as the counterpart to Devil's Sight.)
  • Blinding Light - inside the sun
  • Brilliant Light - staring at the sun
  • Bright Light - a sunny day
  • Lambent Light - the shade on a sunny day, a cloudy day, indoor lighting
  • Dim Light - a moonlit night, a room whose only light is coming from another room
  • Darkness - a moonless night, a cave with a distant light source, a closed room with light coming under the door
  • Pitch Black - no light whatsoever

Here's a list of the sound levels I have. I never did work out exactly how they would be used. I suppose certain actions would produce a certain amount of noise, and the sound level would drop off with distance. There would also be an ambient noise level, which can mask any sounds that are quieter than the ambient noise, making them more difficult to detect and pinpoint.
  • Deafening Noise - huge explosions, sonic booms
  • Thunderous Noise - small explosions
  • Roaring Noise - a loud engine
  • Loud Noise - bustling market
  • Vocal Noise - normal conversation, office or workplace, a piano
  • Quiet Noise - light rain, crickets, frogs
  • Whisper - whispering, rustling leaves
  • Silence - no sound

At some point, I probably would have come up with a similar list for smells. I do think it would be interesting if each creature listed out what senses they had, and how strong those senses were. Imagine a creature whose sense of smell is so acute that they can pinpoint a creature's location as accurately as a human can by sight. Anyway, that could avoid having a gratuitous amount of blindsight with no explanation on how that creature uses their blindsight.

I'm not sure what I'd change about the stealth side of the rules. It seems like no matter what you do, it will still come down to rolling a Stealth check, so it's more just of matter of when you roll, and if other people roll Perception or just use passive Perception. I'd be curious to see how other people have improved upon the stealth and vision rules, it might give me some ideas.