From a sheer flavor PoV all bending should be wis based. It's all about spiritualness and perception. And for other stats: dexterity should be a common stat for forms to use, and for the benders in general. Remember: these are directly linked to four martial arts: water: Tai Chi, earth: Hung Gar, Fire: Northern Shaolin, Air: Bagua. These should be our inspiration as well.
Yeah, but we can't forget about strength and durability in those martial arts either. It's not like we have to worry about anyone ever using Dex as a dump stat for these guys, and Str helps eliminate MAD issues for Earth and Fire, who do quite a bit of 'staying in people's faces'.
*idea: have Deflect Bending be an immediate action penalty to an incoming attack. -10 penalty, not able to get rid of more attacks to fuel this (don't have to get rid of an attack in the first place). This does two things: 1) makes this more "You miss!" while remaining balanced, and 2) gives Octopus a spot in the lime-light. Also, this opens up some room for potential PrCs based on gaining more of these per turn and being able to better use the waterbender's famous defenses.
I'm fine with this. I've personally hated the Deflect Attack mechanic since I first saw it and was just looking for a way to simplify things. Would this mechanic switch be applied to Octopus as well, just giving Octopus the ability to do it multiple times a round?
EDIT: Of course, the problem with this version is that you're not giving up much of
anything for a reliably effective +10 AC Bonus one-on-one at lower levels, since Swift and Immediate Actions, as well as iterative attacks don't show up until later. So its appearance would have to be delayed until at least level 8-ish.
Perhaps it could instill a penalty on your attack rolls for the next round in response for the Deflect action, this penalty lessening with levels? -3 penalty to attack, decreasing by 1 point every 5 levels?
Mm, the more I think about it, the more I think I prefer the reroll version I tossed up, sans the "you can drop iterative attacks to do this more often", which is a bit much. It keeps the exciting feel of an opposed attack roll with a little less paperwork, and it's still a pretty nice bonus. 2d20 take lower (not a completely accurate equation, but one close enough for this) has an average roll of 7, 3.5 less than 1d20. The attacker gets a -2 penalty on the reroll for waterbenders (would get a +2 bonus for fire) which drives the number down further, but the fact that you're not actually taking lower but taking the second result brings the number a bit back up. I'd guess it at something like a +4 average AC bonus in practice.
Also, all of this discussion reminds me just how broken Deflect Arrows is. I think you actually need an epic feat to break it.