I was wondering if something like Biomes or Dimensions (maybe tools too) can be added if its even possible,
for dimensions it could be something like:
to get it you have to make a custom block with a certain property, ones its make you can register it as a portal block type of thing and make it so if you walk in it or right click it, it brings you to a specified dimension (whatever you set it as)
another thought is you could set up a custom dimension property (name it and base it off the normal dimensions, like the end would be a big island (people can expand off it with more portal blocks to "custom outer ends"
for the overworld it you can choose what the normal block would be for the ground and it would well be it.
and for hell, just space with massive gaps in between.