disclaimer: i still prefer to use the Deflect Arrows feat as written. but i've been thinking about some of the other options presented:
i can understand the argument against the auto-deflect of v3.5 considering that the Arrows Deflection magical quality of armor and shields still requires a Reflex save against DC 20 (plus any enhancement bonus of the missile). in v3.0 the feat and the armor/shield magical quality both worked the same way. so why did the feat change in v3.5 while the magical quality remained the same? was it an oversight? if it was a deliberate design decision, then what's the reasoning behind this difference?
in my campaign, the armor/shield magical quality works just like the v3.5 feat as written because i prefer simplicity and consistency.
the thing i don't like about a static boost to AC against missiles (a +4 AC bonus was mentioned) is that it becomes less valuable over time, as it becomes a lesser percentage of a character's overall AC. that +4 seems more valuable when your AC is otherwise 16 than when your AC otherwise is 42. on the other hand, this bonus would figure into your AC against EVERY missile shot at the character. the feat currently negates one missile attack per round that would otherwise hit.
i had toyed with making the Deflect Arrows feat add your base Reflex save to your AC against missiles (Dex and dodge/deflection bonuses are already added to AC, so we wouldn't want to add them twice, just the base Reflex save). this way, the feat's utility scales as the character advances. the drawback is that it still requires you to compute and keep track of yet another AC score. and again, this AC bonus is effective against every missile shot at the character, not just once per round.
it's pretty apparent that the designers wanted the feat to be useful against only one missile hit per round, not to potentially make the character immune to all missile fire.
as a better fix, i like the Reflex save against a DC set by the attack roll because it makes it easier to deflect a poor shot and harder to deflect a well-placed arrow.