With this rule every class would at 1st level +1 bonus to one ability which is important to class mechanics.
This +1 bonus would not stack with racial bonus to abilities(except standard humans), but it would give each race option to get 16 in their primary or secondary statistics.
Each class would have 3 abilities that one of thenm could be raised by +1 with this option.
Reduce point buy pool by 1 or 2 points if you see it necessary.
abilities for each class would be;
Barbarian; str,dex,con,
Bard; dex,int,cha,
Cleric; str,wis,cha,
Druid; con,int,wis,
Fighter; str,dex,con,
monk; str,dex,wis,
paladin; str,wis,cha,
ranger; str,dex,wis,
rogue; dex,wis,cha,
sorcerer; con,int,cha,
warlock; dex,int,cha,
wizard; con,int,wis,