sonic relic
Sure, people could set that manually, but it would be a cool "weather" illusion to wake up and see a thick fog.
Maybe this happens whenever the player sleep during rain/thunderstorm, as often after rain, fog rise.
It would make the sightings more spooky, than currently. As of now, they feel out of nowhere, especially that close.
Speaking of this one, It could be cool to have them also appear but faaaar away, as if it hides in the horizon and observe the player.
visual flame
Unfortunately it's not really possible for them to change the fog with only a datapack and a resource pack. The only way I can think of would be to use a core shader, but that would make it very difficult if not impossible to change the fog at different times/on command
They could change the fog color using a similar setup to SlicedLime's seasons datapack by changing the biomes to a custom biome with a different fog color, but that would be tedious, difficult, and wouldn't be able to affect the fog distance or density, only it's color.
hexed haven
Unfortunately it's not really possible for them to change the fog with only a da...
yes it is
visual flame
Oh really? cool
hexed haven
glsl shaders
visual flame
I didn't know you could change it dynamically
hexed haven
plus shader toggling using a trick I found
visual flame
oh yeah, summon a particle with a specific hex code color
hexed haven
yep
tribal mauve