Auxiliary effects for mental stats

[Bosh]

You mean buff casters further?

Yup, that's the danger of making the mental stats matter more and I'd really like mental stats to matter more so you could make, say, a really smart rogue or a charismatic fighter without nerfing yourself into the group. One way to go about this would be to make mental stats matter more for everyone while hitting casters with a big nasty MAD stick.

Something alone the lines of...

One Hand Giveth:
-Tie Int to skills, jut like in 3.*ed. Have a high Int mod give you more skill proficiencies and have dumping an 8 in Int means that you lose one of your proficiencies.
-Tie Cha to an inspiration/luck system where your starting inspiration bennies are tied to your Cha modifier.
-For Wis not sure Wis needs any more buffing, dumping your Perception and Wis saves already hurts so most everyone tries to avoid dumping Wis.

While the Other Hand Taketh Away:
-Consider returning spell attack rolls to Dex/Str like in 3.*ed.
-For wizards you could make spells known be tied to Intelligence but spell saving throw DCs be tied to Wisdom. You need brains to learn a lot of spells, but raw willpower to power them through people's defenses.
-Do similar things for other casters where they need two stats to be the best casters they could be. Perhaps clerics (and druids?) would get more spells known with Wisdom but saving throw DCs be tied to Charisma. Perhaps the number of sorcery points sorcerers get could be tied to Con. With some other classes you could make them more mad by tying casting to one stat and other class features to another stat, kind of like how 3.*ed paladins used wisdom for casting but charisma for other paladin features.

While you're at it make strength more useful for non-meleers by making an easier to use slot-based initiative system with lots of useful stuff taking up slots (such as certain material components) so that if you dump your strength down to 8 you'll really have to bite the bullet when it comes to useful equipment (to keep bags of holding from being god tier items just have it take too much time to fish stuff out of the bag of holding for it to be too useful in combat).

The basic idea would be to make there be trade-offs for dumping most any stat for most any character. Of course some stats would still be more useful for some classes than others but there wouldn't be so many no-brainer dump stats and someone with an unorthodox set of stats for their class wouldn't so comprehensively nerf themselves.