Save or suck already gradually gets phased out. Semi-fixed DCs like that make them stronger at low levels when they rock hard already, and weaker at higher levels when you can stack many save boosters relatively easily (+6 base, +2 stat, +5 resistance, +5 morale is a total of +18 on a bad save without even trying). Monster save DCs usually cap out at around 25-28 by CR 20, and most player spellcasters are around 5 points above that with level 9 spells at level 20. Of course, optimized DCs will blow that out of the water, but they're extremely variable and go off the RNG no matter what you do to the formula.
Personally, I favour a flat DC 15 + caster's spellcasting ability modifier. No modifier for spell level or class level. That way, save or suck gradually gets phased out as meaningless. At high levels, PCs no longer have "I win" buttons at their disposal, and likewise NPCs can;'t accidentally slay a cherished PC because of a lucky die roll.
This is wrong. The proposed solution solves none of the identified issues. At high levels, PCs still throw around no-save-just-sucks, or optimize their DCs to make up for losing up to 4 points on the high end. Meanwhile, a lucky (or unlucky) roll still means players can roll a natural 1 against that Finger of Death.
Also note that at low levels, you can easily be looking at DC 19-21 saves. Meanwhile, save modifiers are still between -1 and +7. This kind of seals it in favor of the spellcaster.
