After devouring hours and hours of lore on the Deep Ethereal, I'm trying to actually parse everything I've read and listened to into something that can actually be played instead of just acknowledged in-game as, "Oh yeah, that's a place that exists. It's just all wonky. Forget you even heard about it." That's a waste of possibilities, no? 

Here are some of the key challenges and questions I am trying to overcome (Note that we play online over Discord): 

• Limited Visibility - Depending on who you ask, sightlines seem to range from 10 ft. to 60 ft. due to the ethereal fog that permeates the Deep. I tend to vastly prefer running sessions with dungeon maps and/or battle maps, usually made in a Google Sheets template that another DM passed along to me ages ago. However, if visibility is so minimal and if the environment is often prone to change, how does one actually accomplish mapping this out? Narrating it for players to map out square by square like we used to in the old days on graph paper for dungeon dives won't cut it. Similarly, theatre-of-the-mind approaches are likely to disorient and confuse my players beyond what's viable or fun. I've known them all long enough to know they'll get confused and frustrated, say they're too dumb for it, and want to move on ASAP if they don't just stop engaging. That's not great for anyone and just makes me sad and frustrated by proxy. 
• Flight? No gravity? Overall, I'm mechanically fairly prepared for this and for interesting ways it could play out, but I'm curious for any extra thoughts you might have on how it could impact combat and navigation. Hit me with anything that comes to mind. 
• Travel to the Deep from the Border Ethereal - Does it really take only the desire to go there, knowledge of where you wish to go, and the fortitude to survive the experience? 
• Travel in the Deep Ethereal takes ______ amount of time. I can see this getting dull quickly. How does one actually explain that mechanically in a way that has merit and keeps interest? I don't want it to just be punitive. 
• Protomatter, Mists of Creation, and Ether Cyclones. I'm considering different mechanics for how players may influence and be influenced (or harmed) by the volatile essence of possibility within the plane, but again, open-ended question for you all here. What springs to mind? 
• Illusion spells make actual creatures and creations? How does one keep players from abusing this or from it becoming a chaotic arms race of one-upmanship? I believe I saw a table for this before, but I welcome refreshers on the idea or any commentary or feedback on it if anyone has used such a thing. 

I think that covers everything, but feel free to speak up if you have any relevant input on other things I should consider or keep in mind! 

My planned responses/solutions to the above listed concerns and areas of uncertainty are: 

• Limited visibility - I'm considering making several 60 ft. x 60 ft. square "chunks" of potential map bits to plug into what the players will be seeing, probably rolling to determine what's in a given direction. This gives them enough to work with relatively quickly but should still keep an air of uncertainty, tension, and strangeness. I may also simply use this approach for whenever they reach an "island" of sorts in the mists. In between islands, I may use some sort of random encounter table and may utilize the whole, "travel takes a certain amount of time" idea to a degree. Maybe there are a few long rests worth of distance between a location and the next one. I'm also considering the idea of players having to roll Wisdom Saves (probably increasing from a very low DC to a higher one) to stay together on given 60' x 60' squares/navigate to the same place given that willpower seems to be integral to how one gets around, but I'm not sure. Lastly, I don't have much familiarity with the West Marches campaign concept, but is there a way something like that could apply here with regards to building a map or environment? 
• Flight - I've not really given this too much thought, as it's a minor concern. I'm toying with the idea of giving my map(s) a few "floors" of sheets that can be traversed to and exited from at will whenever a player or player wishes it. It could get messy, but it could also add some party-splitting tension, especially if one person wishes to scout a different floor from the other party members. Also, possibly certain kinds of damage (bludgeoning, force, thunder, and psychic) may require targets to make either a Wisdom or Strength save to avoid being pushed to a different map square at random? 
• Travel to the Deep - I'm thinking my players would have to find a suitable location on the Border to attempt the shift to the Deep if my understanding of how that process works is correct. Mechanically, I'm thinking it would be a series of Wisdom saves progressing in DC and in damage dealt for failure to create the experience of having to hold on. It also becomes a matter of resource management, as I'd likely give player characters a chance to hold off and to heal up to attempt again, BUT this also means anyone who succeeded is going to be separated from anyone who didn't, which can create some juicy, tense moments, especially if characters aren't as alone nor as safe as they believe themselves to be during this. 
• Time requirements for travel in the Deep - As I mentioned earlier, I'm thinking I'll just have a list of random potential encounters and that I'll roll first to see how much "distance" is between one island/landmass in the Deep and the next, then will roll for one encounter per each 8 hours or each 24 hours. Additionally, the longer they're in the Deep, maybe the more implications that has for the outside world when they return. A lengthier absence may lead to both more numerous and more outlandish changes when they return. 
• Protomatter - See my earlier mention of rolling for encounters above. I may also use this to make them have to factor in survival factors such as, "Can you subsist on the protomatter/mists you're encountering right now, or are you getting fatigued without proper sustenance and thus accruing Exhaustion levels?" 
• Illusion spells (and other magic) - I'm probably going to just be fairly stringent with how much is possible in terms of players WILLINGLY using this idea/mechanic, but I'm also going to go review any roll tables and extra info I can find. Please do hit me with anything relevant if you know of sources! 

So, have I considered everything I may need to, folks? Am I on the right track, or am I overthinking things? Am I underthinking? Are there other thoughts or prior experiences any of you would like to share? Please, don't hold back at all and feel welcome to say as much as you like! 

Thanks again, everyone!