Matrix Sorcica
Hero
Glad to hear that things will be OK.
Let me add a vote for trying to work with the design assumptions made by the 5e designers as far as possible.
I really feel more is lost than gained by striking out on your own (or emulating a different edition, as it may be)
Consider your core goals for your process: are you primarily trying to fix and patch perceived deficiencies in official monster design, or are you itching to strike out and make your own way?
Myself, I'm squarely in the first camp. I would love CR and progression tables that merely fix the chronic underestimation of optimized high level characters,... and that's it, and then stops there, for a much more official-sounding set of stats.
Assuming your analysis haven't told you the 5e approach simply cannot uphold the challenge, and 4e:ish Solo rules are absolutely required...?
I just wish you'd see that the main issue (at least for me) is how the designers have underestimated the need for every CR 20ish monster to have a "bag of tricks", and to fix that, without getting sidetracked with completely new Challenge Rating data tables or 4th edition rule additions.
Good luck anyway you slice it!
By "bag of tricks" I merely mean that each epic stat block needs to show evidence of at least minimum awareness of its designer that he or she understands what a high-level party can do.
To just take a single trivial example. Any monster that relies on melee damage but can only simply run up to the players to deliver this damage is completely shut down by repellant eldritch blast (not sure on name).
If the monster wasn't epic, this wouldn't be an issue. No harm done in a cantrip shutting down an Ogre, for instance (regardless of level).
If it is only encountered in groups, it isn't an issue. But the MM doesn't contain such information - there's no indication (except the "legendary" status) which foes can and should be the basis of elites and solos. A pretty good start, however, is to ensure no legendary monsters have such a weakness (unless it's explicitly deliberate).
Compare to Forcecage or Wall of Force. To me it's far more okay that a big stupid brute (a really big one, like CR 20 big) is shut down with spells like these than with a mere cantrip. Yes, I know that in both cases what really is shutting down the monster is the action of a high level hero, but still.
That's just one example. The point is to ensure each epic monster has at least some tools in its toolbox, and this is what I meant by "bag of tricks". One trick is "get to melee". Other tricks that have evolved thru the editions include blindsight. We could make a long list.
You can't take these things for granted. Unfortunately, that is. The way the MM is written we're forced to question the system mastery of its designers. And possibly to fill gaps ourselves (this is where this thread comes in! )
Do note I am not advocating standard bag of tricks. Each legendary or epic monster can and should have an individual if not unique trick! That's an important part in the look and feel of their encounter. (Standard tricks only mean the adventurers can and will anticipate them, abuse them and shut them down)
The goal isn't to make the monster or its tricks invincible or unsurmountable.
The goal is to make the players work for their victory; to try different tactics, and to adapt to what works and doesn't work.
Just Repellant Blasting each big bruiser might seem like an innovative solution on first blush, but it really grows old fast. I want (and you want) more from high CR stat blocks!
For instance, take Juiblex, a demon prince of oozes. Things that show care and attention (IMHO) would be to include abilities to ooze out of any non-epic prison (not even a Wall of Force is that ooze-tight?), and some way to quickly appear in the adventurer's midst (it is NOT okay to trivially kite a demon prince like he was your ordinary black pudding).
Zapp
PS. Perhaps a long reply, but do note I was just trying to be clear about "bag of tricks". I really shouldn't have rambled on this long, since it's only a minor issue here. Feel free to focus your reply (when you have time to write it) on the main question: namely what goals are you pursuing?
Soon I will be able to start posting again. If all goes well this weekend I should be able to start posting again next weekend. I will start by returning that which was lost in the great EnWorld crash of 2016 and then the submissions I have received from contributors.
However, in my long absence I have had a lot of time to think about this project and I have decided to change course slightly when I make my second draft of all of these epic threats. I am still working out the details, but one of the things I am think about is to give some monsters multiple turns instead of, or possibly in addition to, Legendary Actions. I haven't decided 100% so I was wondering what you thought. Should I stick to the 5e style Legendary Actions, go back to multiple turns like some 4e solos, a mixture of both? What do you think. Also, any change will be after I first complete all of the entries on the list (plus maybe a few more). I want to complete this train of thought be I make them move to another.