Better Intelligence/Charisma (5e quick fix)

I find Intelligence and Charisma to be of limited use for non-spellcasters (unlike Wisdom, which is very useful for defense). This optional rule gives them more value (notice, however, that there is another quick fix for that).

If you have a positive Intelligence modifier that doesn't grant you extra spells, you can pick one additional language, tool or weapon proficiency for each +1. If you have a positive Charisma modifier that doesn't grant you extra spells, you can pick one additional language, tool or weapon proficiency for each +1. For example, if your modifier is +2 you can get two extra languages, etc.  You can trade three languages (etc.) for one new skill.

(You could do the same for Wisdom, but, again, Wisdom saves are already very useful and the Perception skill is nearly ubiquitous. Also, in RP-heavy campaign you could have additional contacts based on Charisma, but that's is not such a "quick" fix since it involves more tinkering).

Now every one gets a new reason to have better Intelligence and Charisma, without making spell-casters more powerful than they already are - although some multi-classes might benefit a bit.