It would be great if you could dig down in the overworld to where bedrock is now, but instead of reaching bedrock, you can continue to the Nether. Of course, to prevent players from getting there before getting a Nether Portal, they would have to unlock that layer, maybe the bedrock layer is removed when the player builds a Nether Portal and enters it. The End dimension doesn't seem like it could fit joined up, unless you were able to fall from it back into the overworld (presumably without taking fall damage). A better way would be to add a similar Sky Dimension, which was a planned feature at one point, but was scrapped in favour of The End dimension. To make the Sky Dimension unique, there could be all sorts of different structures and creatures that are native to the dimension. The way to travel there could be through the originally planned idea, which was to occasionally get there by sleeping in a bed. To connect it with the overworld, along with being able to sleep again and wake up back in the home bed, you could fall from the Sky Dimension back into the overworld, but not vice versa. Of course, the End/Sky Dimension doesn't need to be connected (which would mean no need for the Sky Dimension anyway) but it would be good to see the overworld's bedrock layer removed after travelling to the Nether once.