I think that the problem here is that a certain ability is too important, if there is a real a choice between more offence (STR) or defence (dex) it works.
Problem there is that just about every class requires one (or at most two) ability scores. And, indeed, classes that rely on more stats are avoided because they lead to much weaker characters (the 3e monk, for example, or... I forget the 4e term. V-shaped classes, or something?)
It's very difficult to ensure that every class requires multiple abilities, and that they are all equivalently reliant on an equivalent number of stats.
My concern is how big the bonuses are. It the highest bonus you can get to attack is +3 at level 10, a +4 from 18 would be stressed over the +2 from 14. But if it is +15 (10 from levels, 2 from weap. focus, +2 from proficiency, +1 from level) then the 18 will not be as needed as much.
Not that I want high bonuses. But piddy +2 bonuses emphasizes max stats.
I agree.
There seems to be a sweet spot where the total a character scores on his attack roll/skill check/whatever is roughly equally split between luck (the d20 roll) and skill (his modifier). Mathematically, it doesn't matter all that much - d20+10 vs DC 20 is mathematically the same as d20+50 vs DC 60, but the former
feels better.
This suggests that typical modifiers, across the life of the campaign, should probably range from -5 at worst (a 1st level Wizard attacking with a sword) up to about +25 at best (a max level Fighter attacking with a sword), with the bulk of the game playing in the +5 to +15ish range.
So, I would recommend the following stat modifiers: 1-2 -3, 3-5 -2, 6-8 -1, 9-11 0, 12-14 +1, 15-17 +2, 18+ +3
Suppose the game then gives a BAB between +1 and +15 (as in 4e), and allows for feats (or similar) to give another bonus between +1 and +5, that should allow for a 1st level Fighter to get approx +5 at 1st level with his sword, rising to about +25 at max level. Which, as I said, feels 'about right'.
(What I don't like about the above is that this seems to put too much emphasis on static modifiers. The max-level Fighter gets his +25 on every attack, whether he just stands there hacking away, or if he carefully employs terrain, tactics, or is aided with magic. Since we don't really want to push the total modifier above +25, because of the sweet spot, that seems to create an issue. I don't currently have a good solution to that!)