Perhaps a different mechanic than Reflex save or automatic?
Perhaps the original flaw in Deflect Arrows was that it did
not take into consideration the skill of the opponent.
IE, there is an equal chance for a monk of a given level (IE,
level 2 or higher) to deflect the arcane archer's arrow of
death, or Chauncey Commoner's xbow bolt at -4 for DEX 2.
In HERO System (AKA Champions) the mechanic is an
opposed check involving offensive combat values. (BAB would
be the same thing as Hero's OCV for our purposes)
If Death Archer's attack roll is a 15, plus another 20 or so
in magic/dex/etc. mods for a net result of 35, Deflect Arrows
might allow the master of Ti Kwon Leep to "attack" an AC
of 35, and if he hits, the arrow is deflected. The new feat of
Snatch Arrows would then say something like "if you hit the
arrow's [effective] AC by 5 or more (or maybe 10 or more) then
you've caught it. Beat AC by 10 (or 20) to throw it back.
(HERO uses point costs instead of feats, but if you were to
translate their Missle Deflection Mechanic, it'd be 1 feat to get
the Deflect Arrows Equivalent, +1 feat per additional thing you
can do. (a feat to catch, a feat to throw, a feat to catch things
larger than arrows, or spell effects, etc.))
Actually, that might be a nice model. Give monks a bunch of
free feats taking them up the deflect arrows chain, such that
while a non-monk would consider deflect and snatch/catch
arrows, only the monk could afford to eventually deflect/dodge
boulders, melf's acid arrow, etc.
Toss in the "attacker's attack roll total is the AC (difficulty class)
the monk must hit (match or exceed) with his attack roll
(deflect skill roll)" and good to go. I'm nowhere near running
a D&D game anytime soon, but even just replacing the reflex
save with an attack roll check would be nice.
Oh, I suppose the monk should get a built-in bonus to the
deflect arrows check, since his BAB suffers compared to fighters.
Perhaps add a fraction of his monk level (like half) to the
Deflect attack roll?