First off- I'm not too familiar with this forum, please notify me if I need to do something or if I have done something wrong.
I'm suggesting a mod that I have recently thought up the idea for-an underground world. I have no idea if there is a mod like this already in existence or if one is being developed currently. If you are intrigued or are wondering if this mod idea of mine would be worth it, I'll list some of the ideas the mod would have...
It wouldn't change the minecraft gameplay much (Added tools, a ton of new creatures, etc.), only that it is entirely underground.
Grassy Dirt would be able to stay grassy dirt underground.
Occasionally biome caves will generate, like desert cave (sandstone from ceiling, sand on the ground in places), and trees will grow underground.
Most of the caverns are connected.
The caverns (especially biome caves) would be partially or totally lit by some kind of means.
Pigs, cows, sheep, and chickens will be able to spawn underground.
Groundstone ceiling, like the Nether.
Underground villages (in a biome cave)
It'd be world type, like superflat and default (would this be achievable...?)
Here are some ideas that I thought of that would not keep the vanilla-y aspect of the mod:
Generated structures like camps and small forts that you could loot.
A new light source called a solarstone. It would be very bright and would light up caves and caverns.
Mole-type creatures that can dig through dirt.
Please give your opinion(s), good or bad. If you are a modder interested in making this into a mod, let me know , I could help with textures/make a texture pack for its feeling of being always underground. Also if you want to add on to the idea, leave a comment!