This is an idea I heard in one of the videos from xisumasays. The idea is that if you step through the portal in the ancient city, you will not get transported to another dimension, but resonated into a different part of the overworld, the Sculk spectrum. This let's you see and interact with all the creatures and features that are hidden in the overworld, but that we can't see or interact with because we live on the wrong frequency. This would also mean that those creatures can attack you. I personally also imagine there being some sort of warden mobs. The lure behind that would be that the sculk sensors together with the sculk shriekers warn the warden population that there is someone in their city. At that point one warden armors up and appears in our normal frequency. This would mean that the wardens in the Sculk spectrum would still be strong, but not as strong to keep things balanced because there are a lot of them. Of course there will also be other mobs with of course special drops. This all with having to be on a special frequency would fit in perfectly with the whole idea of sculk. Seeing that this doesn't really fit in with any of the categories I have just placed it in biomes and dimensions although this isn't really neither of those.