Jewel of the North
First, get rid of saving throws. Use an “attacker always rolls” system where the ability score is the target number for any attacks that target that ability. So sacred flame, for example, would be a ranged spellcasting attack against the target’s Dexterity instead of forcing a Dexterity save.
And maybe go with 4e saving throw (roll 10 or more on a flat d20) for ongoing effects? Yes, that could work. Would ST proficiency still apply? Like, if a class gave you prof in Dex saves, would you add your proficiency to your score? Maybe 1/2 prof, or only +2? Just to be sure to avoid having caster needing to hit a total Dex score of 26 to land a fireball, for example.