First, like I said to begin with, it depends on how you play, but if you do it, it won't break your game.
A fighter with Weapon Focus will use it with every attack he makes,
Not necessarily. I can think of several of situations where the fighter in this example is not willing or able to use the weapon he has taken weapon focus with:
He is unarmed
His weapon focus is with a melee weapon and the opponent is at a range
He decides to grapple
His chosen weapon is ineffective for some reason (it isn't silver/holy/magical/etc while some secondary weapon he has is)
I don't think he is attacked a far greater number of times than he attacks something else. Sure, if he's surrounded he'll take more attacks, but that situation is rare, as the fighter will most often avoid such a situation unless he has Whirlwind Attack, at which point, again, he'll be attacking as often as he's being attacked.
This statement makes several assumptions about how various people play the game. It also assumes that a fighter can choose when he gets surrounded or not. I strongly disagree with that idea. Players in my games often find themselves surrounded whether they like it or not. As a player, I've found myself surrounded plenty of times and hadn't planned for it to happen.
If the party is fighting a single powerful creature, the fighter might very well be attacked fewer times than he attacks. If he stays at a range, or under improved invisibility, or is flying, or is well hidden, or kills every opponent with one hit, he might also attack a lot more than he gets attacked.
When designing feats (or spells or abilities) you can't assume that everyone else's combat scenarios are going to play out like those that occur in your own games. Nor can you assume that certain situations are going to occur more frequently than other situations. you have to base the balance issue off the basic game mechanics that everyone is playing from, because beyond that, everyone has their own way of designing encounters, setting up adventures and creating monster/NPC tactics.
Also, I'm surprised at how many people say they forget to use it. Players in my group often take Dodge, and not because it is a prerequisite either. And we play it by the book and no one ever forgets to use it.
Lastly, I'd suggest that anyone doing that take a second feat (call it Greater Dodge) that allows the +1 AC bonus to all opponents. In fact, I've seen similar feats in various d20 books.
But again, this is such a small issue that it isn't going to throw game balance (whatever that is) out of whack.
Think about it, if giving a
+1 dodge bonus to AC is a game breaker, something else is wrong.
Like I said,
I don't think it would ruin your game if you just made that change.
AS a litmus test for balance, consider this:
If dodge gives a +1 bonus to AC to every opponent, then will any player in your group NOT take it?
If everyone from the Fighter to the Wizard would want that feat, it is probably too powerful. On the other hand, if it takes two feat slots to get that effect, you'll see fewer people wanting to take them (only those characters with a lot of slots, or a real fear of combat will spend the two slots to get the +1 AC to all opponents)
But AGIAN, it really isn't that big of a deal. +1 AC either way is small potatoes.