Not that long ago, someone posted this thread about choosing your casting stat. I left a post on that thread going over the idea of each mental stat providing various benefits, so while you can choose which one to use as your main casting stat, they would all still benefit you. The two effects I feel best about are INT adding more spells known/prepared, and CHA giving you an extra spell slot with a spell level equal to your CHA mod. These could come at the cost of reducing the base spells known/prepared and taking away a spell slot, so that these aren't just straight power boosts, but require at least modest scores in these abilities in order to be at the same place as vanilla.

I didn't really have a good idea for what WIS would do for spellcasting. Here's a list of some of the aspects of spellcasting that could be modified, compiled by another user at the end of that thread:
Quote Originally Posted by GeneralVryth View Post
Paladins and Rangers now have Fighting Styles that grant cantrips with Tasha's so cantrips could work. As for balanced you're talking about what's probably the 4th or 5th pick so it's really adding power instead of options.

Thinking though all the pieces in the spellcasting system you have:
Spell Slots/Points
Spells Known/Prepared
Cantrips Known
Spell Atk. Mod/Save DC
Concentration Mod
Concentration Amount
Up-casting

If generally available you have:
Metamagics Known
Sorcery Points


There are a few options there especially if the system is overhauled a little. Cantrips are likely one good option, but just to offer up a different one what if you WIS mod up-cast levels you could apply to your spells per long rest (or a weaker version you can up-cast WIS mod spells 1 level for per long rest).

You would get CHA giving you raw power, WIS letting you make the most of your power, and INT giving you other ways to use it.
Perhaps WIS might influence your concentration in some way?

There's also non-spellcasting stuff; ideally, mental scores would benefit martial characters, too (and right now they don't, aside from skills and saves). "Martial" is fairly broad, though. They could affect your weapon attacks (attack rolls, damage, number of attacks, crit chance, crit bonus) or your defensive abilities (AC, saving throws, HP, damage reduction, healing, resisting forced movement), or several other aspects of the game (initiative, action economy, resources). Many of these might also apply to casters, so some thought into making them more useful for martial characters should be put in. If casters are already getting benefits from these stats, then they don't need to double up.

On that note, something to be aware of is front-liners will tend to be in the thick of things, and thus making more attacks and taking more hits. Casters, on the other hand, should be staying away from the front line, so when they do get attacked, it's a big deal. An ability that, say, reduces damage taken once per short rest would be more useful to a mage, because they're getting attacked less, while a martial would barely notice the difference. On the other hand, an ability that reduces that damage taken on every hit would see a lot of use by a martial character, but a caster might go multiple rounds without taking a hit, and thus not benefiting. So it probably makes more sense to aim for some kind of bonus that is sustainable rather than something limited by rests. Also, casters will likely have higher mental stats, which is something else to consider.

With that in mind, here's a couple ideas: Crits add your INT mod to damage, demonstrating your ability to identify and exploit an enemy's weakness. You gain a +1 bonus to a number of saves each round equal to your WIS mod, representing your ability to read your opponent and anticipate their attack. You can reduce damage taken by 1 up to CHA mod times each round, representing your determination to keep fighting, even while injured. Note that by going +1 to WIS mod saves instead of +WIS mod to one save, we make the ability more sustainable. It becomes more useful to someone making a lot of saves and less useful to someone who only occasionally makes one save. Same with the damage reduction; to get the full benefit, you have to be taking multiple hits every round. Though these specific abilities are a bit fiddly, and would be easy to forget to apply.

Anyway, what sorts of effects would you give mental stats? Got any ideas I haven't mentioned here?