I'm not sure if this has been posted in the thread already, but why not have a system that works similarly to The Aether mod? from the videos I saw on that mod, it looked as if the new dimension was just enter-able just by reaching a certain altitude.
why not just have a similar system here? when you break through bedrock in Minecraftia you simply go to the nether, when breaking bedrock in the nether you simply go to the end. The converse also applies, when you reach max altitude in the end you o to the nether, when reaching max height in the nether you go to Minecraftia