Anyway, the single-saving-throw is actually very elegant and makes for a very simple implementation: put a column on the class tables, and just make up the categories you want bonuses to apply to, which is pretty much what most of the older rule sets ended up doing anyway. Nonetheless, it's broadly considered to be vaguely unsatisfying, a sentiment which I share.
My best attempt to work with this is to instead reduce saving throw to a matter of HD and ability score, plus ad-hoc miscellaneous modifiers. For characters, saves then break down into the following broad categories:
- Struggle: the character relies on muscle power and training to resist overbearing or crushing, adding their Strength bonus.
- Discern: the character uses powers of observation and deduction to notice harm before it strikes or identify tricks and illusions, adding their Intelligence bonus.
- Brave: the character trusts to faith and courage to carry on in the face of fear, horror and despair, adding their Wisdom bonus.
- Endure: the character relies on their health, fitness, and determination to resist poison, fatigue, or the taint of death, adding their Constitution bonus.
- Dodge: the character trusts their wits and reflexes to avoid dangerous rays, traps, or blasts of magic or dragon fire, adding their Dexterity bonus.
- Presence: the character uses their leadership to carry the day or relies on their sense of self to resist enchantment, adding their Charisma bonus.
One aesthetic decision I made was to make all of the saves into active verbs (e.g., the character struggles against something) rather that nouns representing categories of threat (e.g., the character saves versus poison or death), to place the emphasis on what the character does to mitigate the harm instead of on the threat itself, with the side effect that I'm considering some leeway creatively interpret the categories to put saving throws against certain threats into unexpected categories from time to time, such as by having a character make a discern check to notice the poison in their cup before they swallow instead of the usually expected endure check.
Another option I've considered is giving classes a bonus to saving throws outside of their traditional prime requisites, which they would already be expected to be good at. Perhaps the discipline of a Fighting-Man's training gives even the most naturally cowardly a degree of bravery and a Cleric's ordination gives them more presence.
My final rule of thumb for employing saving throws is that, failing some more specific ruling, characters get a saving throw against any threat that doesn't otherwise involve of rolling with a chance of failure, with the more broad categories hopefully making this fairly easy to adjudicate.