Since Soulsand Valleys are upcoming for 1.16 I think skeletons should receive a different texture if they are spawned in a Soulsand Valley biome.
Skeletons could have a 70% chance to spawn with a "Soul Skeleton" texture which is similar to the normal skeleton texture but the eyes glow the same blue color as the fog in a soulsand valley biome. Additionally the color of their bones could be a slightly darker color. If a spawn egg is used in a soulsand valley biome the skeletons spawned if they aren't wither skeletons would also have that chance to spawn with the skin. You could even take it a step further and add a new variant of skeletons called Soul Skeletons.