Here is a variant saving throw system that I've used in the past. A threat with a save allowed has a level and an ability score it's related to.
Example: Poison N 4th level Con Save.
All or most saves in the game have a die score which must be met or exceeded. there is a lot of room for fiddling here and a high or low score can really set the tone for how deadly and frequent situations that warrant a saving throw are within a campaign.For light fantasy adventure I find a 13 or higher rolled on d20 does it for me. A campaign with a target score over 15 would make things very tough for characters facing higher level threats.
So for a player to save versus that poison N mentioned above they have to roll a 13 or higher to make a saving throw if the target number was 13 for the campaign.
Saving throw rolls are modified by level and ability modifiers.
If a character has a level equal to or within 1 level of the level of the save there is no modifier to the roll.
If a character is 2 level or more greater then the level of the save they get a +4 to their saving throw roll.
If a character is 2 level or less then the level of the save they get a -4 to their saving throw roll.
Ability modifiers are added to the die roll.
If an ability score is a "Prime Requisite" as defiend by the game you are using allow +2 to the roll.
our victims for the examples (using LL ability mods)
Able 6th level Fighter
Str 17, Int 5, Wis 8, Dex 15, Con 14, Cha 5
Zed 3rd level MU.
Str 6, Int 16, Wis 12, Dex 10, Con 15, Cha 10
Able and Zed are exposed to a threat of poison.
Poison N 4th level Con Save.
Able is 6th level so he gets a bonus of +4 to the saving roll and has a Con of 14 for another bonus of +1. He gets a total bonus of +5 to the savign roll.
Zed is 3rd level so he gets a penalty of -4 to the savign roll and has a hardy Con of 15 so he gets a +1 bonus for that. Zed has total penalty of -3 to the saving roll.
Zed and able are beign confronted by an illusion.
Illusion 3rd level INT save.
Able is 6th level so he gets a bonus of +4 to the saving roll. He has an INT of 5 for a penalty of -2. Able has a total modifier of +2 to the saving roll.
Zed is 3rd level so he gets no bonus or penalty for his level. An INT of 16 gives him a +2 bonus. INT is also his prime requisite so he gets another +2 bonus. Zed has a total modifier of +4 to the saving roll.
I like this method becasue it's fairly easy to be a tiny bit fiddly; It reflects charcter levels, ability scores and classes (if prime requisites are a factor). It's also infintely scalable but has a reasonable cap on modifiers.
DMs that worry about "how tough is this save ?" really can relax and key it to the level of the module in question with a minor shift in the score hera and there. 6th level of the dungeon, well it's a 6th level save unless it sdhould be tougher. Bit by a 8HD monster with toxic fangs , it's an 8th level save. Some saves shouldn't get more difficult with dungoen level but DMs should really call that for their own games.
NOTE: Swords and Wizardry has a flat save score for all catagories that varies by level. this score can serve as the target number for making saves.
I'd recomend altering the spread for difference in levels vs the threat and the modifier as well since some of this is built into a save score that already changes by level.
I've used a 3 level spread but the same modifier.
3 or more levels above the threat is a +4 to the roll.
3 or more level under the threat is a -4 to the roll.
so a 5th level charcter would have no modifiers vs 3-7th level saves but would be -4 vs 8th level and above saves and +4 vs 1st& 2nd level saves.