While it's OK for players to achieve the state where their abilities never fail, mobs should never reach the state where they can not challenge players. Thus a saving throw pass is needed to ensure mobs can affect players:
*** Mob damage spikes should be reduced now that there is save based challenge.
1. Chance of MOB Spell Effecting a Player = 1 - (Player Save/(2*Mob DC))
*** No change in the way player abilities affect Mobs --> this is solely to make sure mobs can affect players.
Player Save = 50
Mob DC = 25 or less: 0% of spells land
Mob DC = 30: 16.7% of spells land
Mob DC = 40: 37.5% of spells land
Mob DC = 50: 50.0% of spells land
Mob DC = 60: 58.3% of spells land
Mob DC = 70: 64.3% of spells land
Mob DC = 80: 68.8% of spells land
Mob DC = 90: 72.2% of spells land
Mob DC = 100: 75.0% of spells land
The current abilities (no fail save on a 1) are changed to a reroll (like slippery mind).
Saving Throw Boost (class/racial) ability could give you a reroll within its duration.
Players say insta-kill isn't as good as AoE with DPS, but this change would make them fill a nice role (nuke caster!).
Even single target cc is now useful.
Players still have spell absorption to negate save based challenge for brief periods of time.
This curve probably needs a low level correction: 1 - ((Player Save+X)/(2*Mob DC+Y))
Note that #1 is the most stable with power creep --> and is this probably the best answer in terms of game design with limited resources.