If I was Tread, I see only two options that
really stand out, and those are:
Tactical Awareness and
Hawkeye. Not to say that the other choices are bad, but from a tactical standpoint, these two give Tread a huge fighting advantage (I'll explain more later.) Let's look at each of these in sections, to help me explain my viewpoint.
Timothius:
Deflective Shield (Ex): At 2nd level, an armor master specializes in using his shield to deflect attacks. He gains a +1 bonus to his touch AC, and this bonus increases for every four levels beyond 2nd (to a maximum of +6 at 20th level); however, this bonus cannot exceed the sum of the armor and enhancement bonus to AC provided by the shield that the armor master is currently carrying. This ability replaces bravery.
Burst Barrier (Ex): At 2nd level, a tower shield specialist can use his shield to screen himself from burst spells and effects, gaining a +1 bonus on Reflex saves against them while employing a tower shield. This bonus increases by +1 for every four levels after 2nd (to a maximum of +5 at 18th level). This ability replaces bravery.
Shield bonuses are always an awesome thing to pick-up. Deflect is great b/c there isn't much (or any) that raises your Touch AC (save magical items). While Burst is practically the same thing, but brings with it an increased AC (as I believe Tread only uses a small shield or such, right?).
But the problem with both of these is that you're getting a bonus that is limited in nature. How many creatures have we encounter, so far, that attack Touch AC? Or use burst spells and effects? To my recollection, ZERO. However, gaining a bonus to AC would be nice, so we'll keep Burst Barrier on the list for now.
Timothius:
Bravery (Ex): Starting at 2nd level, a fighter gains a +1 bonus on Will saves against fear. This bonus increases by +1 for every four levels beyond 2nd.
Unflinching (Ex): At 2nd level, an unbreakable gains a +1 bonus on Will saves against mind-affecting effects. This bonus increases by +1 for every four levels after 2nd level (to a maximum of +5 at 18th level). This ability replaces bravery.
Agility (Ex): At 2nd level, a mobile fighter gains a +1 bonus on saving throws made against effects that cause him to become paralyzed, slowed, or entangled. This bonus increases by +1 for every four levels beyond 2nd. This ability replaces bravery.
If all of these could be rolled into one it would be awesome. But alas they have to be taken individually. So unless you plan on taking these consecutively, I don't know if I would even bother. The bonuses are nice, but until we run across these on a regular basis, I would nix all of them.
However, isn't fear a mind-affecting effect? Or does Pathfinder consider these two different things? If this is so, and I had to choose one to consider I'd take Unflinching.
Timothius:
Stand Firm (Ex): At 2nd level, a phalanx soldier gains a +1 bonus to CMD against bull rush, overrun, pull, push, and trip attempts. This bonus also applies on saves against trample attacks. The bonus increases by +1 for every four levels beyond 2nd. This ability replaces bravery.
Weapon Guard (Ex): At 2nd level, a weapon master gains a +1 bonus to CMD against disarm and sunder attempts while wielding his chosen weapon. This bonus also applies on saves against any effect that targets his chosen weapon (for example, grease, heat metal, shatter, warp wood). The bonus increases by +1 for every four levels beyond 2nd. This ability replaces bravery.
How many times have we run across an opponent that uses their CMB? Not many that I can recall. So why build up CMD?
These are kind of specific, but I think Stand Firm has the edge in this one. Most opponents, I believe, will go for the bull rush/trip etc attacks more than a disarm or sunder. Combined these two would be great, but choosing Stand Firm in this group makes the most sense.
Timothius:
Tactical Awareness (Ex): At 2nd level, a tactician gains a +1 bonus on initiative checks. This bonus increases by +1 for every four levels after 2nd level (to a maximum of +5 at 18th level). This ability replaces bravery.
Hawkeye (Ex): At 2nd level, an archer gains a +1 bonus on Perception checks, and the range increment for any bow he uses increases by 5 feet. These bonuses increase by +1 and 5 additional feet for every 4 levels beyond 2nd. This ability replaces bravery.
Here are the two I absolutely LOVE. I don't think you could go wrong in either case. The bonus to Initiative from Tactical is always a plus -- and since Tread is agile, her already good Init gets even better. And this bonus grows as she gains levels. It's a sure fire home run. However, it isn't as diverse as Hawkeye. Your bonus is only to combat situations.
Hawkeye is a good choice, as the bonus to Perception applies to any situation, in or out of combat. Who doesn't love a high perception roll?! And the ranged bonus to archery is helpful, esp since we're on a wide open plain most of the time. Though this too has a negative, as she many not always have a chance to use her bow. So that's a bummer.
So here are the finalists:
Burst Barrier: limited use of the barrier, but you gain a tower shield which means a higher AC
Unflinching: if mind-affecting effects covers fear, this +1 bonus/4 levels has the potential to be awesome.
Stand Firm: a bonus to CMD can be good, if the npcs ever use CMB
Tactical Awareness: Tread shoots first (star wars pun intended)
Hawkeye: Tread sees all, and can shoot further
If it were me (as Gaijin) I'd take Hawkeye. Gaijin uses his bow enough to make the increased range worth while, and the bonus to Perception will always be helpful. But I don't recall Tread using her bow much. She seems more the melee type. In which case the bonus from Tactical Awareness would fit her better, at least at this stage.
So my pick is the bonus to initiative.
[In any case I would recommend you picking up a tower shield. As the bonus to AC will help you greatly, esp when we recall the attack on the Mites and that centipede took you out for one round.]