ok so there are several plugins that could help like how some servers have multiple worlds and custom terrain generation
what if you used plugging to keep items within inventory and use recourse packs like how command block mods use recourse packs, and custom world gen to create dimensions within a server.
using a hybrid between survival pluggins and mini game style pluggins could create boss mob mechanics and recourse packs to create custom models and custom boss drops.
using custom terrain generation, pluggins can do this, could create new dungeons and biomes.
using custom mob spawning mechanics could spawn mobs with blocks on there heads and with the help of data tags and textures could new mobs to inhabit the new dimensions.
I've seen all the necessary pluggins before just take any minigame server as an example for multiple worlds with shared player data, terrain generation can be found on some faction servers, and boss mobs, well thats just repurposing plugins from minigames of course!
the point is the necessary pluggins are all there but is anyone willing to do the work? Is anyone actually interested in this concept?