I'm of the opinion that 4E and 5E have unnecessarily high HP counts. I think that if every PC had hit points within a range of like 20 to 35 for the entirety of their careers it would make creating balanced challenges easier. Granted, this would need to be coupled with reworked rules for hitting 0 HP and dying... but I'd rather that then trying to grind down PCs that have 100 HP or more every fight. But that's just my feeling on the matter and I know is not close to being a widespread opinion.
So, no growth in HP, no growth in damage, no growth in AC, no growth in features, no growth in skills...
It sounds like you want people to play 1st level characters from the word "go" until the campaign ends. What's the point of that?
Huh. So when I said
"I think that if every PC had hit points within a range of like 20 to 35"...
you read this not as referring to hit points, but rather referring to hit points, damage, AC, features, and skills. Interesting. I wonder what other posts I have made in the past that apparently had more buried in them than the actual words I used.
I assumed, given you did not mention anything else, that you didn't want to add any growth not present in 5e currently. Which, yes, means essentially no growth (3, perhaps 4 points at most) in AC and a bunch of other things besides.
The designers specifically and explicitly said that HP and damage growth were intended to be one of the only forms of statistical growth in 5e. If you remove that, what is left? Skills. Perhaps features (though, frankly, unless you have spellcasting, most of those features suck.) That's about it. Hence why I said what I said.
I find this exchange interesting in that level up = HP Bloat seems ingrained in the mentality of many D&D players and GM's so much so that they do not mentally separate increasing HP from character advancement.
DEFCON 1's HP bloat solution is nothing more than the static HP model used by Runequest and many other RPG's. And it is a solution that works very well. 5e's bounded accuracy is tailor made for a mod like this. Skills and features are the Primary modes of advancement in other RPG's.
The features (feats/class abilities) in 5e actually do get good if you have fixed PC hp in the 20 to 35 range. Right now all they do is keep the PC on par with their level appropriate HP bloated monsters. 5e's main issue is that they made the traditional attack bonus and skill point advancement tied to the same number. When both of those things used to be one of the primary ways classes were differentiated from each other.
Separate out attack bonus per class, and dole out skill points as a PC levels up - BooM - the issue is solved. And those would not be game breaking hacks to implement...
If every PC has a lower and narrower band of hit points... you can look at any creature and their average damage for each attack and know at a glance "this one will need to hit probably four times against any PC to knock them out" and "that one can one-shot any and all of the PCs with a solid damage roll". And that's when armor class and higher defenses and the like come more into play.
This is how most every other RPG not D&D works. It is much easier to eyeball 'encounters' when PC HP is essentially a static number. HP Bloat is the main reason why WotC D&D has always had scaling issues at high levels.
DnDone could solve a lot of High level play and CR scaling issues simply by going back and adopting the 1-2e AD&D solution: stopping HP Bloat around the 8-10 level range with the rest of advancement occurring as normal.
However, Level = Moar HP is so ingrained into what WotC D&D has become that it will not only never happen - Such a solution will not even occur to the designers or play testers...
I mean, I see new (and by new I mean “have only played 5e”) players complain all the time that D&D has so many books!
Part of the problem there IMHO, is that WotC does everything as shiny hardback releases when in reality most of those books are Adventures and Settings most tables have no need or interest in...
There aren't actually that many "game" books, but a newbie just looking at the shelf just see's a bunch of hardbacks that all look alike at first glance.