The planet would have a surface with many different ruins of a dead civilization and the climate would be freezing due to clouds blocking the surface from sunlight. The previous sentiant inhabitants accidently wiped themselves out when a new fuel source for an experiment went catastrophically wrong and the very air of the planet was slowly poisoned with toxic gases... the surviving creatures eventually adapted to be resistant to this enviromental change but the sentiants died.
The planet before it's end would of had villager inhabitants as the previous inhabitants that eventually evolved to a technological revolution of advancement without any players wrecking their world of rich resources or hording materials, only to die out and players who land to find the shell of a former utopia with advance techs they can scavenge or repair over time. The atmoshpere was almost exactly like the overworld but due to the toxins that infected the very air, it will poison players who breath it but it is survivable with poison resistance of some sort. The villagers who once thrived on the world had no access to mystical componants of any form, so no potions and enchantments to make the standard poison resistant stuff on the overworld... no wonder they died out
The local mobs can be poison resistant forms of their overworld counterparts with slightly more HP. The villagers were working on a specific technology that players can find the blueprints to and complete: Aether Walkers, their last ditch effort to save themselves from extinction... but they couldn't complete them due to missing materials that matched the needed requirments to allow them to work on their world, mystical materials. Players can bring certain mystical stuff like potions or nether objects to complete the project.
The Aether Walkers primary function was to allow the locals to walk on the clouds that recently blocked out the sky, normally entities would pass through the clouds like air but with the Aether Walkers equiped the clouds would act like solid blocks to that villager. This would allow them to move to the skys, leaving the tainted surface behind to save themselves... but since they failed, players can finished the boots by scavenging certain required materials from the ruins and adding certain componants from the homeworld. Wearing these would allow players to walk on the clouds and explore, the air up there is clean and breathable due to the dense clouds containing the toxins bellow so it's a good area to inhabit if you have the proper gear to do so.
These clouds could have special dungeons scattered around them, maybe the locals once had the required componants but due to them moving to all technology over time they lost such things to complete the modern recreation over the thousands of years they advanced? Either way, one things for sure... they didn't make it out alive and players have a post apocalyptic wasteland to explore xD
These advance villagers had one other effort to save themselves, it was three colony ships titled the S.S. Emerald, S.S. Diamond, and Farlander. The Aether Walkers idea was conceived by desperate souls after the homeworld lost all contact with all ships for various reasons. The S.S. Emerald crash landed on the overworld and the villagers that were lucky enough to survive the crash saw how similar this new world looked to the records of there worlds former past self, and so they swore off technology in order to start anew and avert a possible repeat of the past... thousands of years past until they forgot their orgins and became the villagers we players see scattered in rural villages around the world.
The S.S. Diamond succeeded to an extent, it was primarily a exploration vessel for new worlds to inhabit and it was their advance genetic engineering technology on board that allowed them to create villagers that could survive all sorts of enviroments easily. In their search for a new world, they planted 'seeds' of various genetically altered villagers on many worlds they visited on their travels thus leading to 'moon villagers' and such sub-species. Sadly the ship and it's crew did not have a happy ending... a stray asteroid and a lack of attention for less then a second had a hole blown in the hull, killing all on board due to violent decompression. Some say the ship is still floating in the deep darkness of space, forever forced to orbit the local stars asteroid belt while still holding it's precious genetic and enviromental technologys for those who seek it out... the journey will be perilous but the rewards worth the effort.
The Farlander was the the only true success to their mission of colonization, despite their communications being fried on enterance of their new homes atmoshpere. The planet they landed on is pretty much a jungle version of their home world with scattered plains, oceans, and underground caverns full of life. The world has an electomagnetic field that stops wireless communication and can severly damage a ship that isn't properly equiped for enterance.
The world has a seperate dimension from the one the villagers landed on similar to how the overworld has the Nether or End... the difference is this planets dimension is the creation of a temporal flux in the planet itself due to a special material making up it's core and an event in the planets past creating a temporial feild that encompasses the entire world.
The 'dimension' is where the villagers originally landed, and the destruction of their ship in a effort to erase their roots and start over without technology to try and avoid another possible planetary destruction essentially caused a planetary wide event without their knowledge. The explosion in combination with the special core super charged the gravitational field, leading to the planet being tangibly split into two different time periods weather they know it or not. The surface players land on is the 'future' and is essentially full of ruins of settlements, large cities, and various leftovers from the villagers, who eventually died out over the course of 5000 years on this new world thanks to a local plauge they couldn't cure... without their ship they couldn't get off world so they slowly died out.
The surface the villagers live on is the 'past' and the time period is before the plauge hit when the villagers were thriving in their large cities. Players can find randomly spawning rifts on both surfaces that will send the player to the other time period at the same coords the rift they traveled through was located and anything players do in the 'past' will have effects on the future world, for example: A player builds a home or decides to live with the villagers in the past, that same home will appear in the future as a ruined structure at the same coords it was originally build in the past and any block removed or just moved in the past will have it's future counterpart disappear or relocate! This will give players the feeling the world is 'alive' in the time based sense and cause some to be extra careful when exploring the past world.. or you get the players that just want to see the world burn and just don't care and set off TNT galore in the past to see the effects in the future xD
If any entity with at least five mobs of itself other then players is brought to the past and left there(aka, mobs not natural to the dimension), the future will start spawning that mob to simulate a change in past mobs that make up the future ecosystem. If a player doesn't like these new future counterparts, they can undo their existence by traveling back and killing all the 'seed' animals in the past, all mobs will count as being killed by "<mobname> faded from existence" to give a special message for named ones. This system can even work with modded mobs so say I leave a crimson cleric from Thaumcraft in the past... that's gonna be one deadly future o_o
