I prefer Fort/Reflex/Will as saves because it highlights the character's strengths and weaknesses. I also prefer the difficulty of saves to depend on the attacker, in symmetry with attacks. Basically the 3.5 system, except that the DCs usually depend on the attacker's bonus, not the spell/effect level per se. It must be admitted that the 4e system is simpler, though, and I can see why some people would prefer that.
I wouldn't mind having a save for every ability score, as long as there weren't huge disparities in what is targeted. That can be an issue even with 3 saves, and it is harder to fix with 6. That said, ability scores that don't matter should die its death in 5e.
I think a dial that might work for save or die effects is as follows. Define such effects in a method similar to 4e. The template I would use it that such effects take a certain number of rounds to complete. In most cases there would be at least two conditions: a lesser one which keeps the character in the action, and the terminal one which removes the character from the combat entirely, whether it kills them or not. A progression from start to end is also fine. The effect is ended if the character succeeds on a single save vs. that effect before the terminal condition. (More than one save required is also compatible with this idea, I just think it gets a little out-of-hand.) Note that since the progression is determined by rounds, and not the result of individual saves, we don't have any wonkiness where a player might actually die faster by being granted extra saves which he fails. To turn the save-or-die dial, you merely start the effect at a particular round.
The Medusa (following 4e) might look like:
Round 1 - Slowed
Round 2 - Immobilized
Round 3 - Petrified
With the save-or-die dial at 1 you start with slowed, and as long as you make a single save before you're petrified all is well.
With the dial at 2 you start immobilized and pretty much have 1 round to save yourself.
With the dial at 3, you have classic save-or-die, except the attacker rolled rather than the player. (This is a big reason why I prefer symmetry between saves and attacks, especially if using this complexity dial.)
This also works if you want to screw the player up front, but not necessarily permanently. An alternate version of the medusa could very well be
1 - Petrified (or mental actions only?)
2 - as above
3 - Petrified (permanent)
The player may or may not be able to make saves on his own, but perhaps his essence is sufficiently present for others to help him do so. Or give the wizard time to dispel the effect and so on. The point is, it is very flexible for these kinds of attacks.
In addition, for effects that are bad but not quite to save-or-die levels, one might make 2 rounds versions. As long as all monsters adhere to some standards (like "instant-death" effects take no fewer than 3 rounds), the game runs smoothly on all these settings. Another mechanical restriction is that the effects of later rounds can only depend on state, not choices or consequences specific to an earlier round of effects, as these may not have actually occurred for some settings of the dial.
I could also see "half-dials" for those who mostly want one thing but don't mind the occasional exception. Basically, if the attack crits the effect is harsher than normal. Either skipping a round or increasing the save DC come to mind. This could also be built into individual attacks, of course, or the DM could adjust the dial for a single boss. The important thing is the DM will always know what he or she is getting.
As an aside, if effects typically progress on the defender's initiative (4e) rather than the attacker's initiative (3e) you might need to twiddle with the precise timings, but the basic idea is unchanged.
If I were to run a game I'd probably be pretty comfortable with complexity dial 2.5, with harder saves on a crit. And since medusas are scary, maybe they crit on 18-20.
Here is an example of how an iconic spell might work:
Power Word Kill
No initial attack roll. Bloodied creatures (or whatever) take -5 on saves.
1 - Moderate psychic damage
2 - Quite a bit of psychic damage
3 - Death (if bloodied), otherwise none.
(In general I think you ignore round 1 entirely if you start on round 2 or 3, so that early damage never occurs. This is cleaner as a rule, easier to perform in play, and avoids the balance issues of piling on multiple rounds worth of stuff while actually giving the target fewer rounds to deal with it. They're already dealing with an even more imminent death, there's no need to rub it in.)
This spell looks pretty scary to me at all dial settings with respect to the amount of swinginess one presumably wants in the game. And with the dial at 3, very evocative of the classic.
