Main Idea:
The Mod would be called The Shroud Update. Pretty much it goes like this. You [and whoever you're playing with] are criminals and are put inside a giant walled in biome of dust and ash. Practically what remains of a once flourishing flower biome and a lake. The Ash Forest Biome has taken over the surface and the Deep Sea Trench Biome has taken over the lake. Your objective is to survive and find a way out. Doing so, you discover ruins of past settlements of previous criminals who failed to escape. You find background data on The Shroud and who they are.

Main Mechanics:
The Daytime is very bright and warm. Ash falls from the sky near dawn and dusk but it has no special effect other than visual. Ash Skeletons only spawn at Night, Dustmites spawn anytime of day. The Night is much darker than normal nights in the Minecraft World. It is very hard to see as there is a dark ash cloud that surrounds you almost like Silent Hill Fog. It is very cold at night, staying too far away from light sources for too long will freeze you to death. Day time is recommended to be the time you explore, craft, build and mine. When Night comes, stay at your base and guard it as best you can.

Biomes:

The Ash Forest:
The Ash Forest is covered in mostly ash blocks and a few Ash Wood Trees here and there but enough to be satisfied. Ash Skeletons and Dustmites appear here.

The Deep Sea Trench:
The Deep Sea Trench will be mostly stone, sand and deep sea stones and sand.

Mobs:
So far only three mobs exist in the unfinished mod. Ash Skeletons, Dustmites and the boss variant of the Dustmite, The Mother Dustmite who is 7 blocks tall.

The Mother Dustmite has to be located and killed if you want their loot, they will not seek you out and kill you.

Ash Skeletons do not like light and will stay away from it as much as possible. They also have no step sound, so they are very quiet unless they breathe. Their breathing is raspy and ghost-like and they do not breathe often. They are resistant to fire and because of their weapon, deal rare fire damage.

Dustmites are attracted to light, though easy to kill they break doors and windows until they break the light source. But if they see you, they will go after you as well before they attack the light source.

Materials:
New materials will be added with the mod. Such items are either found or taken from mobs.

Land Materials
Dust Clumps are found only by killing Dustmites. As are Dustmite Eyes. Both are needed for a new recipe.
Charred Gems can be turned into Flame Rubies.
Ash Wood can be found by finding an Ash Tree and mining the logs. Easy as that. Ash Wood can be turned into every block you could turn any other wood into. Doors, boats, planks etc.
Charred Bone can be collected by killing Ash Skeletons. You can turn Charred Bone into normal Bones and then turn it into Bone Meal for crops.

Sea Materials:
Purifier Blocks can be found deep in the trench. They can be turned into 4 purified powder. Purified Powder will be used in numerous recipes to turn something like Ash Blocks into Dirt Blocks.
Stone can now only be found underwater. Digging on the surface will only lead to a layer of bedrock 15 blocks below the surface. [It's to stop you from leaving the biome by digging]
Sand can now only be found underwater.

Crafting:
A few new crafting options would be new light sources, crafting benches etc.

A New Lantern would need to be crafted from the Purification Powder.

Weapons:
New materials calls for new weapons.

Flame Ruby can be turned into the assortment of tools and weapons like Swords, Axes, Shovels, Pickaxes and Hoes.
Tridents can also be crafted from Flame Ruby as Drowned cannot be found in the trench.

Armor:
Same with new weapons comes new armor.

Flame Ruby can be turned into armor. Helmets, Chestplates, Leggings and Boots all contain fire resistance.

The Way To Freedom:

The only way to escape the "Prison" is by crafting A Golden Key. The Golden Key will unlock a gate on the south side of the location. Opening it, you have to fight your way through guards and escape the facility. That will lead to the end of the mod. To craft a Golden Key you'd need a vast assortment of things.

A currently unmade sea creature at the bottom of the trench will serve as a boss. Likely a giant sea serpent or eel. This boss will drop 7 gold ingots upon killing it. Use the 7 gold ingots to craft the Golden Key then try and make your way back to the Southern Gate.

If you made it this far, I'd be surprised. It's a lot of information to take in. But if you did make it this far and are interested with helping me and @StripesJ make the mod, go ahead and comment or send a friend request. Something like that.