I've seen this idea float around before but math seem a little off on the DC and is only going to further divide those that are proficient and those that are not and still punishing both.
So let's take a 1st level Wizard with 16 INT.
Current rules have his Spell DC at 8 + prof (2)+ Int mod (3) = 13 DC
Your version would have DC at Ability Score (16) + prof (2) = 18 DC
Now let's say this Wizard hurls a fireball (I know, he's only level 1) at a group with a Fighter (Dex 10), Rogue (Dex 16), Wizard (Dex 12), and Bard (Dex 14).
Class | Dex Score | Current Mod | New Mod | Current Dex Save | Nex Dex Save | | Old d20 roll to hit DC | New d20 Roll to hit DC |
Fighter | 10 | 0 | 0 | 0 | 0 | | 13 | 18 |
Rogue | 16 | 3 | 6 | 5 | 8 | | 8 | 10 |
Wizard | 12 | 1 | 2 | 1 | 2 | | 12 | 16 |
Bard | 14 | 2 | 4 | 4 | 6 | | 9 | 12 |
As you can see EVERYBODY is now worse off and those that are not proficient in a save are essentially screwed. The fighter goes from a 35% chance of success to a 10% chance of success.
Now the curve levels out a little bit once a stat is maxed at 20 (by level 8 for most classes if we assume all ABI's go to main stat) for CHARACTERS, but for monsters with stats above 20 the DC's will quickly be unreachable by anyone not maxed in that stat and proficient as well and almost all save will require a natural 20 to succeed.
Level 8 | | | | | | | DC 16 | DC 21 |
Class | Dex Score | Current Mod | New Mod | Current Dex Save | Nex Dex Save | | Old d20 roll to hit DC | New d20 Roll to hit DC |
Fighter | 10 | 0 | 0 | 0 | 0 | | 16 | 20* |
Rogue | 20 | 5 | 10 | 8 | 13 | | 8 | 8 |
Wizard | 12 | 1 | 2 | 1 | 2 | | 15 | 19 |
Bard | 14 | 2 | 4 | 5 | 7 | | 11 | 14 |
*Only saves on natural 20.
Here is a level 13 party vs Adult Blue Dragon CR 16:
Level 13 | | | | | | | Breath DC 19 Dex | Breath DC 28 Dex | Wing Attack DC 20 Dex | Wing Attack DC 30 Dex |
Class | Dex Score | Current Mod | New Mod | Current Dex Save | New Dex Save | | Old d20 roll to hit DC | New d20 Roll to hit DC | Old d20 roll to hit DC | New d20 Roll to hit DC |
Fighter | 10 | 0 | 0 | 0 | 0 | | 19 | 20* | 20* | 20* |
Rogue | 20 | 5 | 10 | 10 | 15 | | 9 | 13 | 10 | 15 |
Wizard | 12 | 1 | 2 | 1 | 2 | | 18 | 20* | 19 | 20* |
Bard | 14 | 2 | 4 | 7 | 9 | | 12 | 19 | 13 | 20* |
*Only saves on natural 20.
Even the maxed out Rogue falls from 50% success on Wing Attack to 25% success. Even the proficient Bard with decent Dex only saves on natural 20's anymore.
Not a bad concept but the math needs to be better figured out. No one will want to play a game that they fail at 90%+ of the time.