I like the idea since it allows for more diverse character concept (the charismatic barbarian, the strong monk, the intelligent fighter, ...) without making you feel like you're not making full use of your class abilities. It also mostly solves the problem of racial stat bonuses pushing players to certain race/class combos, in a better way (IMO) than just allowing moving the stat bonuses.
If you want to stay close to the current math, the easiest thing is to start out with a +3 bonus at level 1 which increases to +4 at level 4 and +5 at level 8. But I bet it wouldn't break the game either to do what you propose and have it increase at levels 3/7/11/15/19.
If you want to stay close to the current math, the easiest thing is to start out with a +3 bonus at level 1 which increases to +4 at level 4 and +5 at level 8. But I bet it wouldn't break the game either to do what you propose and have it increase at levels 3/7/11/15/19.