In Pathfinder 2e, what would be the effects of making Fortitude, Reflex, and Will static defenses like D&D 4e?

Question

I am a longtime 4e player who has been running a PF2 game for about 8 levels now. One of my biggest frustrations with it is that Fortitude, Reflex, and Will are sometimes used as saving throws and sometimes used as a static defense e.g. Demoralize using the target's will DC. This makes some level of sense from a roleplaying perspective but in terms of game design it isn't all that fun or sensible. Additionally, I generally like static defenses more as it means players do not need to always be ready to roll dice on anyone's turn and can pay better attention to combat, or go to the restroom when not their turns, etc.

What would be the effect of always using DCs rather than saving throws? Would the math change significantly in terms of likelihood of effects working? How would one go about it? My instincts say that the default DC level of 10 + modifier is probably good for the defensive end, but for the character or enemy using an attack, should I use (save DC - 10) or something different?

Answer 1

Fortune/Misfortune Effects

Many affects allow the player to re-roll saves, either for good (Fortune effects) or ill (Misfortune effects). All of these would need to be revisited on a case by case basis to see how they would interact with your change. This includes Hero Points, as @brandon mentions, but also many important feats and abilities:

• Halfling Luck, which is probably the best feat line for Halflings (similarly, Cat's Luck for Catfolk)
• Incredible Luck, an example of one of many class feats that'll be affected (Swashbuckler in this case), or Grit and Tenacity for a lower-level similar feat (for Gunslingers)-
• Bit of Luck, an example spell
• Black Cat Curse, an example misfortune affect that'll be affected (also affecting Catfolk)

Most of the above can be pretty easily converted to instead make the enemy re-reroll appropriately, but there's all sorts of small things you'd need to watch out for - for example, does Incredible Luck grant the circumstance bonus to the player's save DC? Or do you apply it as a circumstance penalty to the enemy? (I recommend the former).

The Bigger Problem: Math

Again, this can be accounted for, but it's not as simple as just switching from the DC with +/- 10. It's a little counterintuitive, so here's an illustration: Bocephus the Goblin has a +10 Reflex Save, and Abraxias the Evil Wizard casts Fireball on him with a Spell DC of 20. Results for the normal rules (die roll, save result):

1. Critical Failure (due to natural 1)
2. Fail
3. Fail
4. Fail
5. Fail
6. Fail
7. Fail
8. Fail
9. Fail
10. Success
11. Success
12. Success
13. Success
14. Success
15. Success
16. Success
17. Success
18. Success
19. Success
20. Critical Success

This means there's: 5% Crit Fail, 40% Fail, 50% Success, 5% Critical Success

However, if you have Abraxias roll at +10 vs Bocephus's Reflex DC of 20, you get:

1. Critical Success equivalent(due to natural 1)
2. Success Equivalent
3. Success
4. Success
5. Success
6. Success
7. Success
8. Success
9. Success
10. Fail Equivalent
11. Fail
12. Fail
13. Fail
14. Fail
15. Fail
16. Fail
17. Fail
18. Fail
19. Fail
20. Critical Fail Equivalent

That means now Bocephus has a:

• 5% chance of critical failure (the same)
• 50% chance of failure equivalent (instead of 40%)
• 40% chance of success equivalent (instead of 50%)
• 5% chance of critical success (the same)

That's a huge swing in the favor of Abraxias. If you apply the same rules to players and monsters, it's at least seems equally penalizing to both sides, but many monsters already have a higher Spell Attack to compensate for this, so the players will still be extra penalized. If you want to make the math work, you could try adding 12 to Bocephus's Reflex Save to get his DC... but even then, as @Ruse points out, that alone doesn't solve it. Even using 12 instead of 10, Abraxias would critically fail (aka Bocephus would critically succeed) when he rolls a 2 or 1. So you'd have to make critical failures for this roll only happen when failing by more than 10. Again, not impossible, but I prefer to minimize complications at the table, and having two different DCs with different rules (one for normal things, like Demoralize, another for inverted saves like getting Fireballed) for each save seems awkward. (As a side note, getting to be the one rolling the die is an often overlooked advantage of Spell Attack spells.)

I've never met a player who doesn't want to roll dice

I've not played Pathfinder with everyone on planet Earth (yet), so I can't say for certain... but every player wants to roll dice. It's fun. I wouldn't recommend rulings that cause players to roll less dice, especially outside of their turn, when player engagement is at its most difficult. It's not really as fun to just watch the GM do everything as it is to get to roll dice in response to the GM doing things.

Answer 2

Hero Points Would Be Much Less Powerful

By making all saving throws into checks against the target's DC you eliminate the option for player characters to reroll important saves. This makes player characters experience the worst effects in the game more frequently, and would make the game harder.

As a side effect this would increase the likelihood that players spend their Hero Points on skill checks and attack rolls, as there's less to save them for. Paired with being unable to reroll saves this should make fights more swingy, increasing the odds of things going very well or very poorly.

Beyond that, there's the thematic effect of making things feel out of the player's control/luck⁠—the difference between "That enemy's so powerful" and "I can't roll for beans". This is particularly pronounced in PF2e as critical failures and critical successes are based on more than just natural 1's or 20's with the DC +/- 10 mechanic.

Math

Moving the die roll from one character to another wouldn't change the math here, assuming that abilities which only trigger on rolled saving throws are still allowed to work in some way. Using DCs of 10 + save modifier and attacks of save DC - 10 would be appropriate. substantially changes the math, thanks to @ESCE for catching that the math is different using other values than I considered in my initial answer, a +11 bonus vs DC 20.

Of particularly interesting note, there is no way to adjust the DCs or bonuses to fix this thanks to how meeting a DC is a success and that critically failing happens at 10 below that DC. There can only ever be <= 45% chance of failure on any check in PF2e, while there can be a <= 50% chance of success, so this situation cannot be fixed without something like meeting the DC being a failure.