A very simple mod that adds the Create: Aeronautics End Sea to the Overworld and Nether 40 blocks below the lowest y level where you can place blocks, which is -40 for the end and nether and -104 in the overworld. Now the End can't hog the void sea for itself. and the aether, I guess...
Does not add anything to break bedrock, that's up to you. (although i DO have a mod for that.)
i'm unlikely to add more dimensions, but feel free to suggest some on the discord.
here's the discord, btw.