I've been dissatisfied with saving throws as normally presented for a while now, primarily because their progression curve is just so confusingly uneven - in LotFP, for example, they generally jump up by two every three levels, but sometimes it's three, and sometime's it's five. Secondly, the periodization - if saves are another method of making characters more survivable the more player time is invested in them, I'd prefer them to scale up in tandem with that time investment, rather than at every third level.
I also really liked Gus L.'s idea of decreasing Death Saves. It's a crutch that helps out starting characters but automatically falls away as they (and the player) gets more experience, it telegraphs the idea that you can't expect to be saved from the consequences of your mistakes as you keep playing, and finally, it works pretty well as an implied aging mechanic.
So, thinking about it further, I decided to go with a four-save system - Body, Reflex, Mind, and Luck. The first three is the basic three-save system. Body protects you from poison and exhaustion. Reflex protects you from triggering traps. Mind is rolled against magical effects, both live and device-based. Luck is your "Save vs. Death."
Here's the basic progression:
In the same vein of a decreasing Luck score, I wanted each save to behave differently. All of them slow down after level 7, in keeping with the general idea of higher levels providing diminishing returns. (Originally I tried slowing down after level 10, but the progressions I used lined up better at level 7, with all four saves lining up for levels 7, 13, and 19.)
Therefore, the Body save starts high and rapidly increases - +1 every level, followed by +1 every other level - reflecting your character starting a bit doughy but quickly becoming much tougher, in time to protect against the more powerful poisonous monsters they're likely to face at higher levels.
Reflex starts high, but only gets +1 every other level, followed by +1 every third level. This way, you start out comparatively well-protected against traps, but it doesn't increase as quickly as the dangers you face - you'll be forced to rely more on your own wits and attention to your surroundings than the saving throw.
Mind saves take the opposite tack - starting fairly high, but getting a +1 only every two levels at first, then increasing to +1 every other level. I wanted this to reflect the relative rarity of magic-using enemies in the early levels, but with an accelerating scale of improvement as they become harder for players to simply avoid. It also makes in-game sense to me, with the characters gaining better abilities to fight magical effects as they are brought in more direct contact with them.
Luck takes the same scale as Mind, but decreases. -1 every third level, then -1 every other level.
With a basic progression figured out, I whipped up four specific save progressions:
Divine, of course, can be used for clerics, paladins, etc. Arcane gets applied to magic-users and illusionist types. Physical for fighter types. "Lucky" for thieves and halfling types. I'm working on a seven-class system for Veil, which applies the Lucky progression to the Pioneer (a frontier-styled Specialist), the Arcane to the Magus and the Illusionist, the Divine to Assassins and Clerics (due to Assassins being based on the original religious Hashishin order) and Physical to Fighters and Barbarians.