There was 'sky dimension' before beta 1.9 .
It was replaced to 'End'.
I hope it replaced as world type.
If it would be added, it generates same things as default, except ocean, bedrock, etc.
and it should have its own biome.
The aether is a different dimension, this would be normal minecraft terrain generated floating. And, if would have all the normal biomes, only "floating". Minecraft already had this GREAT feature back in time. Example: