I guess that is a fair point that generating the lava ocean above an entire other biome would be a pretty daunting and difficult to design thing to do, I guess I wasn’t thinking about the design practicality when making this suggestion.
I still think the order of biomes is a good idea, because when I first saw the pictures and video of the soul sand valley, I got pretty excited, and imagined it as this spooky cave that made you feel like you were slowly crawling though the rotting corpse of some part of the nether. If it would be more of a cave than an area that just has lots of soul sand, I would have to be in the lower areas of the nether for it to work, being separated from the giant caverns of the upper part of the nether with a huge lava ocean that you couldn’t be sure what lay underneath. This would make finding the biome a lot more special, seeing as the only way to get down there is through either digging through a pillar of netherrack jutting through the ocean into the lower depths of the area, or through exploring another cave that just as surely leads you down there. The soul sand valley wouldn’t even be visible from up above.
Ghasts don’t need to be exclusive to the biome at all, either. Honestly, I think that the biome shouldn’t have ghasts at all, they should just stay in the spacious nether wastes.
Lastly, for the concerns with portal linking, they would link the exact same as always. The only thing I think would change is where is generates in the nether when making a new portal in the overworld, as to not make someone spawn in the valley or right next to the lava ocean. When making it in the nether, I would either link to a portal in the overworld or make a new overworld portal in the same way it a works in 1.15.
also, there may be new biomes announced soon for the new update, if so I will update this.