I know you've got plans of your own, but I figure it wouldn't hurt to mention my own internal concept for it. I've kinda talked about this before, but my visual conceptualization of it has evolved over time. This isn't me trying to push ideas or suggest anything, just putting what might be cool on paper for my own sake. It goes without saying, but I'm not Paul so this doesn't reflect any future developments, for anyone reading.
The way I envision it, the spirit world would kind of be a world made of energy and forces rather than matter... At least, that's what the appearance is for me. Exchanges of energy and impacts would radiate and cause fluctuations in things around it, creating audible sounds, shifting light, effectively causing /tiny/ shockwaves, and maybe even affecting gravity a smidge. I'd imagine the spirit world being something constantly in flux, the terrain itself looking more like energy flowing, shifting, being expelled, influxing... Physics might not always work the same way, and what terrain and sky that there is, wouldn't necessarily have any consistency, in my eyes. Maybe sometimes you'd be moving faster, other times slower, perhaps color shifts a bit because light stopped flowing the same way, or you start sinking into the ground because suddenly those forces keeping you and the floor separate stopped working. Maybe you start changing size or getting squished or stretched on occasion; ideally you'd be able to see the warning signs or onset of this by looking at how the energy is flowing in the area.
My conceptualization of the spirit world feels... loud. Maybe sounding somewhat like NASA's voyager recordings, but you know, better sounding, more impactful, more present, but still sslightly uncomfortable. It doesn't feel like it would be a nice place. There could be these occasional, massive shockwaves from gods know what that mess everything up intermittently, causing unpredictable effects to you or your surroundings... that aren't significant enough to be a huge resource draw or danger, of course. Maybe the energy surrounding blocks would flare out a bit from shockwaves, like they're being broken down by it. There would probably be more solid areas... areas 'closer' to the overworld, maybe? Or if it's some sort of realm of gods, perhaps those would be present in these more solid places. I dunno. It just feels like matter as we know it shouldn't have much of a foothold there, and that what matter is there would be sparse and fragmented.
Perhaps there's something... I'll call ether decay, wherein physical objects in the spirit world are eroded or converted by the plane into energy(giving any physical visitors a time limit where they get weaker as they go on, though magic users would be able to sustain their bodies much longer). So the spirit plane would be like our dark matter, pretty much. I also thought that the spirit world should place much, much more importance on your magic and life energy stats, or inverting magic and physical stats in some way. Perhaps mages are made stronger in this interpretation of the spirit world, but physically rather than through magic, with greater resilience against ether decay included. Maybe high constitution players would have to rely more on magic? This inverted relationship, perhaps, would be due to magic energies behaving differently between the two planes, and maybe it would be more difficult to use magic in one plane after extensive practice in the other.
Oh! And I just thought of something else. Back to the idea of massive shockwaves, imagine the shockwaves occasionally converting the energy type of areas. This is assuming that certain biomes will correspond to a specific energy type, of course... but imagine if every once in a while a random seed was generated and used to mix up the element type of different regions. I'm thinking it could be done through a world state, but that would also necessitate the shockwaves hitting everyone at once... which isn't necessarily out of the question. Time wouldn't work the same there as it does in the overworld, presumably. Anyway, back on the topic of world states, I'm thinking you could have a set of blocks tuned to change to specific, mutually exclusive elements with each new seed(so all elemental block types remain unique between shockwaves, but actively switch elements when a large shockwave hits). That way, there just has to be a transition animation and you just have to switch out the on touch events and whatnot between them rather than changing each blocks value individually; meaning world-scale changes can actually be processed! ... presumably. I'm not a game developer after all. It just seems like it would work.
Terrain wise, it would be unpredictable. Sometimes you might get circular platforms, other times, asteroid fields. Maybe sometimes there are just sheer walls and flat plains for no reason. Other times, maybe there are chunks that look somewhat like the overworld, or a cave system, or... an inverted cave system, or still lifes of tempests and spiralling galaxy-like structures, or random theoretical structures and statuary, maybe a menger sponge for instance... each of these different archetypes blending into eachother seamlessly as a beautiful yet terrifying abstract.
And yeah, that about sums up my ideas relating to the spirit world. Well, the spirit world as a raw concept, at least. Some are less possible than others, and some are sillier than others, not that there is any expectation of this being implemented, but this concept was definitely neat to explore in my head. Hopefully it was neat exploring my head a bit in this thread, for you.
EDIT: Building onto the idea of certain areas being more solid in the spirit world, maybe the solidity would be dependent on the life density in the area? So, if you cause an area to become overwhelmed with miasma and get destroyed, perhaps that lack of life and consequent lack of a connection to the physical world would make it so certain areas are like quicksand or even just voides without first creating a 'life bridge'.