Nether Tunnel to Overworld

At the bottom of the World you could sometimes find a flat portal that functions like the End Gateways. It would teleport you to another flat Gateway to the Nether. The portal in the nether would spawn you into a long winding tunnel that would eventually slowly mesh with the nether blocks and you would find yourself at the top of the Nether.

This would function based on the seed, finding nether caves that are close to the top already and meet specific requirements, like dungeons. Then it would generate this structure. This structure would remove the top of the nether and replace it with a large bedrock encased cave system that blends the over world with the Nether. At the top of the cave would be the Gateway, when you jump into the gateway it would function like the End gateways that it looks for the most suitable block to place the player at the other side. The side in the overworld would generate in the exact spot it was in the nether but X and Z times 8. The portal on the Over world side would generate a small cave that winds up till it meshes with a cave of some kind.

The reason to add this feature, because it has been an idea that has been around for a very long time since they first added the nether in Minecraft. Adding this feature can also give player a way to travel between the Nether and the Over World without having to build portals. There would be natural ways of traveling back and forth if your willing to explore enough and find one of these portals.