Let's start with the save versus Death Ray, 'cause it's easy. I don't have "death rays" in the new game, so there's no need for any such save. There are spells that'll rip your heart out of your chest, but no death rays. That's an easy chop.
Now, let's move on to Poison.
So many amazing things you can learn from this internet thingy when you start doing research. Things like the distinction between poisons, toxins, and venoms. Things like which creatures of the animal kingdom actually have venom (there aren't all that many). Things like what various poisons will do to the human body (and what they won't do).
Paracelsus wrote that everything is a "poison" if taken in the wrong dose...but for our purposes, we're not worried about excessive eating or drinking. We're talking about things like snake bites and "poison gas" and giant spiders and the occasional poisoned blade or arrow.
Let's look at normal animals first...and we can probably skip the platypus and poisonous fish. Things like snakes, scorpions, spiders, and lizards are staples of the dungeon-delving genre. These creatures...real life animals...have venom that is capable of killing adult human beings. Capable of killing a human, but death is not a certitude...which is probably part of the idea behind saving throws. For sure, it's a retroactive justification for the idea. A successful poison save (or fortitude save) means the creature didn't inject enough venom to kill the hearty player character. Instead, they just take "damage."
But how much damage do these animals do in real life? A snake can bite you 20 times (through your boots if not plate armor), but it's doubtful such bites would severely dent your "adventuring ability" (i.e. diminish your hit points). Spiders only inject venom on an "attack bite" (against prey they consider potential food)...spiders have a "defensive bite" when scared or threatened that contains no venom (they apparently have superior control of how much venom they inject)...but would such a venom-less bite do even a single point of damage?
To me it makes more sense to model venom in terms of actual damage...if a normal (non-adventuring) human has 1D6 hit points, than D6 damage would adequately model the random possible deadliness of an envenomed attack. A "6" roll would mean a dosage capable of felling a strong human, and a "1" would be a relatively mild (if still painful) shot.
"But my fighter has 20 hit points, JB! How could my character ever be killed by a king cobra?"
Well, you my lucky friend, have simply become the beneficiary of "dramatic license." I believe I previously mentioned that my new fantasy heartbreaker is not about scurrilous rogues, but bona fide Heroes. And heroes just don't "auto-die" from the bite of a black widow.
|How many poisonings can a halfling survive?|
*ahem* The venom from larger monsters can be extrapolated from the way we model normal animals. A giant viper, might have fangs the size of daggers (and inflict similar damage from a bite) and inject a larger amount of more virulent poison...doing more damage.
Because that's what poison does to humans...it damages them. And if it damages them enough, it kills them. That's pretty easy to model with hit points of damage.
Remember the origin of the poison save? A hypothetical "giant spider" in Chainmail that took out a piece that failed its 2D6 roll of 8+. In Chainmail, there aren't any "hit points"...you're either removed from the board, or you're still active. D&D (and its descendants) have done away with the one-shot-one-kill of Chainmail and instead provide individuals with hit points. Use 'em.
[also remember that Chainmail's giant spider doesn't poison anything but the rank-n-file pawns, instead melee-ing like a lycanthrope when engaged with other fantasy characters: heroes, wizards, and the like]
But what about reduced capability? Getting the shakes from that rattlesnake venom and whatnot? Didn't DND3 do a great job with that ability damage shtick?
Well, first off notice that the older editions don't really worry about this "reduced capacity" nonsense, at least with regard to poison (other than shadows, I don't think there's a monster in the core B/X books that reduce ability scores, though I may be forgetting something). Reduced capacity can be reduced to "color" just like any other wound ("that spear attack is dripping blood...you're walking with a limp now...your head really hurts after that orc clubbed you; might have a concussion"). It doesn't need to result in an actual mechanical penalty. The game is hard enough (well, without Monte Haul magic gathering and superhero-like feats and abilities)...but even if it isn't, what's to say that when the shit hits the fan and the PC's ass is on the line, he/she won't overcome the "shakes" of the poison or the pain of the wound. Your character will feel awful for hours, but in combat situations you pull yourself together, juicing on adrenaline and your own heroic grit.
You don't need penalties.
Poison gas? Well, what's it do? Is it some sort of nerve agent that kills you? Or (my son's favorite superhero trope) "sleeping gas?" Or is it some sort of medieval tear gas equivalent? If it's non-lethal and you spring it, then you should face its effects. If it's lethal (nerve gas and such is incredibly lethal) then maybe it shouldn't be in the campaign (*ahem*).
But assuming it makes sense to include such a lethal trap, then on a case-by-case basis you might have a die roll to "save." Not to resist the poison, but a chance to hold one's breath when the trap is released. Heck, treat it like an encounter: roll for surprise. Characters that aren't surprised can have the chance to hold their breath (smart players will probably volunteer to do just that) and retreat from the area. Give the thing a range of lethality (so PCs on the outskirts might not be wiped out). If the denizens of the area are aware of the trap (because they created it/set it themselves) they probably have some sort of antidote lurking around (and nearby) for any such accidents.
You really don't need this saving throw.