A submarine ride would be great, but I think there is not enough space for such a ride. If it would be built, I want to see a Nautilus themed ride, like it was at WDW and not the Finding Nemo version in Abaheim.
Nevertheless I hope that the Nautilus will open the whole day again and not just a few hours. We always missed it during our last visit. I love the theming a lot and I hope it will never be replaced. It just fits perfectly into Discoveryland.