An interesting development from another recent thread of mine. The subject was on trying to develop a rock-paper-scissors type of interaction between casters using different casting stats. I came up with something that wouldn't apply to spellcasters specifically, but to any character based on their mental scores. I thought I'd crosspost it here, since it relates to the discussion.
Quote Originally Posted by Greywander View Post
I don't think I'm looking for something quite that strong. Making it too one-sided means there's no longer any skill involved, only bringing the correct type to the table that beats your opponent's type. I'm thinking something more along the lines of just gives a slight edge to the one with the stronger type.

Here's a rough example of something like what I'm thinking, though I have no clue if this is remotely balanced. We can even separate this from being a specific type of caster and instead just make it to do with ability scores (so any character can do these, not just casters).

Let's say that INT mod times per long rest, you can substitute a d20 roll with either a 15 or a 5 (plus whatever modifiers). This includes rolls you make, attack rolls made against you, or saving throws made against one of your abilities. So INT characters specialize in knowing ahead of time what the result will be, and planning around that.

Now let's say CHA mod times per long rest, you can reroll any of those same rolls. Keep in mind the INT feature must be used before rolling, while the CHA feature is used after, so using the CHA feature will overwrite the INT result, rendering it useless.

Finally, let's say WIS mod times per long rest, when those same rolls use multiple dice, you can choose which one gets used. This applies after the CHA feature, again rendering it useless (or worse than useless, if the reroll was a better result for the WIS character). However, the INT feature doesn't feature a second die, or any dice at all (though you could treat it as a single die showing a face value of 5 or 15), so the WIS effect can't be used.

Again, no clue if this is balanced, but it demonstrates the kind of effect I'm looking for. While these effects aren't limited to casters, casters would have the highest mental stats, and thus the most uses, and these features would likely see the best use with high level spells (or defending yourself against such).
Quote Originally Posted by Greywander View Post
Here's a couple examples. First, some non-spell examples that can apply to non-casters.

You make an attack with a weapon. You use your INT to force the die roll to be a 15, plus modifiers, practically insuring a hit unless the enemy's AC is very high. But sometimes a 15 isn't enough...
You make an attack with a weapon. You have disadvantage because you are blinded, but use your WIS to choose the better of the two results, overcoming your disadvantage through superior experience.
You make an attack with a weapon. You roll poorly, so you use your CHA to reroll, getting a better result through sheer determination.

You run into some ball bearings and make a DEX save. Basically the same as above, but now applying to a saving throw instead of an attack roll.
You make an Athletics check to climb up a cliff. Halfway up, you need to make another Athletics check, and don't want to fall. Again, same as above, but now applying to an ability check.

You cast Disintegrate. You use your INT to force a 5 on the saving throw, practically insuring a failure, unless the enemy's save bonuses are very high.
You cast Disintegrate. The target has advantage on the saving throw because they took the Dodge action, so you use your WIS to choose the worse of the two results.
You cast Disintegrate. The target rolls high on their save, so you use CHA to force a reroll.

An enemy mage casts Dimension Door to escape, so you Counterspell. You use your INT to make the Counterspell check a 15 plus modifiers, but the enemy uses CHA to force a reroll, nullifying your 15.
An enemy mage casts Dimension Door to escape, so you Counterspell. You roll well on the Counterspell check, so the enemy uses CHA to force you to reroll. You use WIS to take your original result.
(Technically, the way I wrote it out, you shouldn't be able to use these abilities on someone else's ability checks, but I do think it should apply to Counterspell. I'm just not sure how to properly frame the rule to work that way.)

Basically, it comes down to:
[You/your target] makes a(n) [ability check/attack roll/saving throw] and [you/your target] uses INT to force the result to be [5/15]. [You/your target] uses CHA to reroll, nullifying the INT effect, so [you/your target] uses WIS to take the original result, nullifying the CHA effect.

Actually, the more I think about it, the more I like these. INT is kind of in the weakest position, since it needs to be used first. Everyone else can just wait for you to use your INT effect, then nullify it. But INT is pretty reliable when you do use it and it doesn't get nullified. CHA is in a pretty good position, since you can wait to use it until after you get a bad result, but a reroll isn't guarantied to succeed. WIS is interesting, because you generally can't use it most of the time (if the roll doesn't have advantage or disadvantage), but what it does do is let you clutch through those bad rolls more easily, making you more reliable in a tight spot.
Now, one issue is that even though these benefits would apply to everyone, they would disproportionately benefit casters who invest more in their mental stats than martials do. However, I do quite like these, and they would be pretty useful to martial characters who are able to invest in their mental stats. Perhaps similar bonuses could be created for the physical stats to help balance out the benefits between martials and casters. Perhaps a STR bonus that benefits attack rolls somehow, a DEX bonus that does the same for ability checks, and a CON bonus that works similarly for saving throws (this would also help address DEX being the god stat, and CON being underwhelming). I'm worried that this could get too busy, though.