Perhaps you could start by telling us WHY all characters should conform to the same array (or small set of arrays)?I've never used arrays for chargen in my past campaigns; my long-time group of players are point-buy hounds. I am now thinking about using them in the future.
As a DM, I like the idea of everyone agreeing to the same array, but that's very unlikely to happen, and I like to give players interesting choices in chargen. So instead I am considering giving them three or so arrays to choose from.
I can name one exception using an example you gave. You said that it is justifiable in some cases to use a 20 in your attack stat. As a weapon user, that gives you a +1 over the 18. If you take a +3 proficiency weapon, that gives you another +1 over the +2 proficiency weapons. That makes expertise not needed on the 20 primary character with the +3 because he's has the same hit probability as the 18 with the +2.
I have to chime in on the side of, "why limit the players' options?" Let them use any of the legal point-buy or array options.
You have enough to worry about as a DM without making niggling changes to how characters are built.
No. If a character is justifying four dump stats in order to be really good at hitting, he's quite likely to want to be ... wait for it ... really good at hitting. Expertise might be the second or third feat he takes, because if he's willing to pay for a starting 20, he's likely to be willing to pay a feat to get that extra +1.I can name one exception using an example you gave. You said that it is justifiable in some cases to use a 20 in your attack stat. As a weapon user, that gives you a +1 over the 18. If you take a +3 proficiency weapon, that gives you another +1 over the +2 proficiency weapons. That makes expertise not needed on the 20 primary character with the +3 because he's has the same hit probability as the 18 with the +2.
There's seldom a mathematically better option by fourth level. It's just too obvious for anyone to ignore when it becomes +2.there's no mathematically better option by the time you're looking at your 8th feat (14th) and expecting it to jump to +2 at 15th.
Again, I'm open to counterexamples.LoL, you just completely ignored the premise I put forth in your "feat tax" garbage. You counter by assuming everyone who spent the 20 will also spend the feat. Sure, many will, but it's a build option period. I would argue the feat is most often a far "better" option for what I try to do than buying that 20, but I'm certainly not advocating it's the only way to do it "right".

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.