There's a tweak a'coming for my m20 system, or at least one that I'm noodling around, trying to find out exactly how I want to address it. Prompted by discussions I've read, and one or two I participated in, on the apparent invulnerability of PCs in 5e, it occurred to me that higher level PCs in my game have too many hit points, and will tend to thus prolong combats to an undesirable level. It is not, however, just a high level problem; I think I've been way too cautious overall, and given much too much thought towards low level play at the expense of high or even mid-level play.
I'm reminded that in B/X (I checked my pdfs to be sure of all the details) you only get one hit die per level; you never start with a "full hit die" at 1st level, the CON bonus modifiers are much more modest than they were in d20, and at "name level" you stop gaining hit dice and only gain a single hit point after leveling up (assuming you were human; demihumans just capped out their levels entirely at name level.) And arguably, B/X was generous compared to OD&D. While I don't have OD&D around to check, using Swords & Wizardry as a proxy (and I think it's the same, or very nearly so) everyone's hit die is a d6, but you just roll it, and unlike in B/X; you don't always get a hit die when you gain a level; some characters, like magic-users, only get a new hit die every other level. The progression is otherwise a bit all over the map.
While I do think it is highly desirable to flatten out that hit point progression somewhat, I think that it is not desirable to shift it upward. So starting out a 1st level character with only 3-4 hit points sucks and should be discarded, but I don't want to end up with 9th and 10th level characters—or even 5th or 6th level characters—with so many hit points that combats become tedious, overly long, and therefore boring. And I think as it stands, that can happen a bit too easily. The flattening needs to happen by pulling hp totals up on the front end, but leaving them about the same on the back end. Also; keep in mind; even very powerful enemies, such as a balrog... er, excuse me: baal-rog—only has 40 hit points. If any but the highest of high level characters has hit points equivalent to or more than a baal-rog, then I'm not doing it right. The most hit points any monster on my monster list has is 100—the angel. But that's more as a point of reference; you're not really ever meant to fight one. Not meant to imply that the baal-rog is a good choice for a war of attrition; I'd probably go for the 75 hp sea serpent there, but the sea serpent is meant to be a big monster and therefore it makes no sense for PCs to ever be in that hit point range.
The way it was structured now, a character could; if he rolled an 18 for STR, picked the Neanderthal race, and had really good hit point rolls, easily end up in the mid-70s for hit points by 10th level. Something was wrong. I miscalculated that aspect of the game.
The proposed way hit points will work going forward is that you start with your STR score as your hit points (not your STR score + 1d6, as it was in the outgoing system.) Every time you level up, you gain +2 to your maximum hit point total. That incredibly high STR Neanderthal at 10th level now has 44 hit points (which reminds me; I need to make it more clear if your STR score is adjusted by leveling up, you do retroactively update your hit points to reflect that as well.) A truly average STR character (10) would, at 10th level, have 28 hit points. A real weakling, a scholar who focuses on learning magic, for instance, could be closer to 20. I would imagine that for most characters, hit points would top out at 10th level in the upper 20s or somewhere in the 30s. This seems consistent with the range in S&W and not terribly unlike B/X (a little lower, but that's OK.) And it is certainly a lot flatter, which I really like. If an average character starts out with hit points in the low teens and ends up, ten levels later, in the mid to upper 30s; that's a heckuva lot better than starting out at 3 hit points and ending up, ten levels later at around 50, as in B/X. Heck; an average 10th level Rogue in Pathfinder would have closer to 100, and a 10th level Barbarian could easily be brushing up on 150 hit points. That's insane. And, of course, since in my game the damage ranges and enemy hit point ranges are lower, it only makes sense that PC hit point ranges be lower as well.
Cons: there's no variability. The idea of rolling your hit points was always half the fun. I did think of having hit dice still make an appearance; a hit die of d4 would give you very close to the same average result, mathematically, as a flat +2 (+2.5 to be more precise. So at 10th level, you'd have only 5 more hit points on average by rolling.) If I do that, I will also eliminate the notion that you can roll twice and take the better of the two results, though, because that skews the math upwards and we start getting way out of the desired range.