I'll use my own group of player characters as my example, as that is where the problem currently is for me.
Background
Here are the defense values of my group of level 16 adventurers.
Bard
AC 30, F 21, R 28, W 29
Rogue
AC 30, F 22, R 32, W 28
Fighter
AC 36, F 31, R 27, W 28
Swordmage
AC 34, F 26, R 28, W 28
A typical monster at this level had +20 against at NAD and +22 against AC. The chance to hit against NADs varies from 100% to 45%. The chance to hit AC varies from 65% to 35%.
Formulating the Problem
I am not unhappy with how AC works. I hated the 3E situation where AC outgrew attack bonus to the degree that most critters only hit on a 20. AC is also where most of the damage comes in, and my group is actually taking too little damage to be exiting at the moment. So the question could be; how to get NADs to the range where AC is now.
The spread in AC is six points. The spread in NADs is 12 points. While I can see that a greater spread is desirable, I think this is too wide. The chance to hit against NADs varies from 100% to 45%. This seems a little too high (indicating NADs are too low). About 35% to hit the strongest NAD (a 10% change or +2 to the highest NAD) makes sense.
It seems to me than, for my party at level 16, an increase of about +6 to the weakest NADs to +2 on the strongest ones, resulting in a spread of 8 points with a hit probability from 70% for the weakest to 35% for the strongest. Extrapolating this, I think a +10 bonus to the weakest NAD and +3 to the strongest is reasonable at level 30.
If possible, I want a solution that is easy to express in the character generator.
Proposed Solutions
Change class bonuses to NADs: Right now, most classes get a +2 bonus to the NAD connected to their strongest attribute modifier. This increases the trend to NAD overspecialization. If all characters instead got a +2 to all NADs, that would even out the distribution. This results in a +2 bonus to the weakest NAD.
Increase more attributes: Instead of increasing 2 attributes twice per tier, characters are allowed to increase all (or at least three) of them. This results in a +3 bonus to the weakest NAD over 30 levels, about +2 at my party's level level.
Moderate level bonuses to NADs: Give out a +1 bonus at levels 5, 10, and 15. This results in +3 at level 30, +2 at my party's level. This bonus is easy to play with; it can be doubled to +1/5 levels very easily, for example.
Applying all three of these fixes to my current party would increase the weakest NAD by +6 and the strongest NAD by +2, which is pretty close to the target figures.
My team would end up with (rough estimate):
Bard
AC 30, F 26, R 31, W 31
Rogue
AC 30, F 28, R 34, W 31
Fighter
AC 36, F 33, R 32, W 32
Swordmage
AC 34, F 31, R 31, W 31
Final Thoughts
I'm not saying this is the way to go; this is a rough first draft and if I knew what to do there would be no need for discussion. The part I am most skeptical about is the increase to all attributes at every level. Maybe not including the all-attribute modifier and instead increasing the level modifier is better, but this also increases the strongest NADs. The attribute increases plays havoc with feat prerequisites and makes starting with an 18 in an ability score even better offensively, but by making the lowest NAD competitive and thus increasing the value of a bonus there, this might still be balanced. And perhaps my goals are set too high too; maybe NAD attacks are supposed to be generally stronger?