This feat is notorious for its poor wording. The “+100%” phrasing is completely unique within D&D 3.5e as far as I know, for example. Ultimately, I can’t imagine any other interpretation here than adding again the number subtracted from your attack rolls, and it does have the nice feature of specifying the “normal” damage from Power Attack which means that features like the frenzied berserker’s supreme power attack that already give one-handed weapons 2:1 returns don’t get doubled to 4:1, but instead go to the 3:1 you would normally expect from D&D’s multiplication rules.

But then there is the line you haven’t quoted:

If you use this tactic with a two-handed weapon, you instead triple the extra damage from Power Attack.

No bizarre “+100%” in sight! But also we have lost the useful reference to “normal” and now it is multiplying “the extra damage from Power Attack,” whatever that is for you. This is going to get us in trouble, you can just tell already.

So you are tripling the extra damage—not tripling the penalty applied. The problem here, well the first problem here, is that “the extra damage from Power Attack” is “twice the number subtracted from your attack rolls” when attacking two-handed. Worse, since “the extra damage from Power Attack” is calculated as twice the penalty, but isn’t itself subject to any multiplier, arguably the repeated-multiplication rules don’t apply, and that gets you a 2×3=6 rather than 1+(2−1)+(3−1)=4. So instead of 2:1 returns on Power Attack, you get 6:1 returns on Power Attack. Or maybe you get 5:1; it’s impossible to say since it’s worded so poorly. Plus, ya know, I suspect what they meant to do was give you 3:1 returns, but of course they didn’t say that.

And that would combine quite nicely with, say, the supreme power attack feature of the frenzied berserker, who was getting 4:1 returns to begin with. Now they’re arguably getting 8:1.

On top of those issues, this is only the Power Attack bonus damage. The result is added to the rest of your damage, and that gets you your full damage... which might be multiplied again, e.g. with valorous. This effectively multiplies your multiplier, which is exactly what the multiplication rules try to avoid, but since two different things are being multiplied, the multiplication rules don’t actually come into play.

So for the example: 2d6+1 damage from the weapon itself, +6 for Strength, and the −6 attack penalty for maximum Power Attack results in double that for +12 damage from Power Attack without Leap Attack. Thus 2d6+19 is the baseline for all interpretations, and valorous doubles that for 4d6+38.

With the 6:1 returns, we are instead looking at Power Attack bonus of +36 (six times the penalty, triple “the extra damage from Power Attack” which would have been +12). Using 5:1 brings that down to +30, which is somewhat better, but not, ya know, great, when what they probably meant was +18. Note that +36 is nearly what valorous was giving the entire attack before. Now with valorous, we’re looking at a total of 4d6+66—of which, 52 comes from Power Attack.

It may not be a bad idea to try to eliminate the multiplication of a multiplier here through houserule, but note that the Power Attack bonus damage isn’t the only case of this: the bonus damage due to Strength also has a multiplier, +1½×, which is also being doubled by valorous. This, unlike Leap Attack, has strong precedent in the rules. The “fix” would be to apply the multiplication rule individually to all sources of damage, like so:

\begin{array}{r} 2 \times ( && 2\text{d}6 && +1 && +1\tfrac{1}{2}\times 4 && +3\times 2\times 6 & ) \\ = && 2\times 2\text{d}6 && + 2\times 1 && + 2\times 1\frac{1}{2}\times 4 && + 2\times 3\times 2\times 6 \\ = & [1 \\ && +\left(2-1\right) \\ & ] & \times 2\text{d}6 & +[1 \\ && && +\left(2-1\right) \\ && & ] & \times 1 & +[1 \\ && && && +\left(2-1\right) \\ && && && +\left(1\frac{1}{2}-1\right) \\ && && & ] & \times 4 & +[1 \\ && && && && +\left(2-1\right) \\ && && && && +\left(3-1\right) \\ && && && && +\left(2-1\right) \\ && && && & ] & \times 6 \\ = && 2\times 2\text{d}6 && +2\times 1 && +2\frac{1}{2}\times 4 && +5\times 6 \\ = && 4\text{d}6 && +2 && +10 && +30 \\ = && && && && 4\text{d}6+42 \\ \end{array}

But this is very-definitely a houserule, and I’m not convinced that it is good (I mean, good luck calculating that for every attack!), even though it “enforces” the idea that you’re not supposed to get to mulitply multipliers.

So barbarian gives you Strength and Constitution; bear warrior gives you both but more. Assuming you still want to avoid multiclassing, feats are your only real options. Those probably should look something like this:

• City Brawler bonus feats: Improved Unarmed Strike, Two-Weapon Fighting

• Human bonus feat: Improved Grapple

1. Extra Rage

3. Power Attack

6. Improved Bull Rush

9. Shock Trooper

12. Improved Two-Weapon Fighting¹

15. Greater Two-Weapon Fighting¹

18. Versatile Unarmed Strike²

¹ These feats require a lot of Dexterity. It’s entirely possible that you won’t have and won’t want that much Dexterity. In that case... more Extra Rage, I guess.

² Or anytime earlier if you feel you’re running into a lot of such DR.

That gets you five rages per day, your unarmed strikes can deal a variety of damage types, and at the end you get a lot of attacks in a grapple. But your grappling will be only mediocre, your attacks are not going to do particularly high damage, and Shock Trooper is only thrown in there because it’s a decent feat that will make you at least not entirely useless when you can’t grapple.

Some multiclassing options

If you can multiclass, you should. Barbarian does not offer much at all from 3rd to 7th level. If nothing else, you can take fighter levels to free up more feats to take Extra Rage, so the one significant bonus at Barbarian 4 can actually be done better without barbarian levels.

Fighter

You don’t really need feats all that badly, but several of the feats you’re taking are fighter bonus feats, which means you can take them with a couple levels of fighter and take Extra Rage instead. You can also snag Combat Reflexes if your Dexterity isn’t awful, so you can take an attack of opportunity, and with Improved Grab, start a grapple then-and-there.

But fighter should really never be taken for more than two levels. A feat per level is OK, a feat every other level is awful.

Ranger

The advantage of ranger is that you can get Improved Two-Weapon Fighting as a bonus feat (ignoring the Dex requirement, even!), and then take Favored Power Attack from Complete Warrior, which at least makes your Power Attack useful against a couple of types of foe, and the Distracting Attack variant from Player’s Handbook II, to help the rogue out against enemies you can’t grapple. You also get a lot of skill points and a couple of spells, to make you less of a one-trick pony. The idea here would be Barbarian 1/Ranger 6/Bear Warrior.

Horizon Walker

Horizon walker is a core prestige class (i.e. technically it’s from Dungeon Master’s Guide not Player’s Handbook), but you can squeeze two levels of it in before qualifying for bear warrior. It requires Endurance and 8 ranks of Knowledge (geography), so it works best with at least three levels of ranger.

The big advantage of horizon walker is the desert terrain mastery. That gets you immunity to Fatigue, which means you can end your rage early without becoming fatigued until the end of the fight. Plus you’re immune to other sources of fatigue and exhaustion results in fatigue, instead, which is nice because fatigue is bad and exhaustion is awful.

For the second level, underground’s 60-ft. darkvision is almost certainly your best choice; a +4 competence bonus to a skill is just not all that exciting.

After you finish bear warrior, returning to horizon walker long enough to get a planar terrain mastery means you can take the shifting mastery, i.e. the ability to use dimension door once every 1d4 rounds. Dimension door is far from a great spell, but it is teleportation which can be difficult to come by for a martial character. The cavernous mastery’s 30-ft. tremorsense isn’t bad either, though by that level you really need to have gotten into the air.

Ideal build is probably just Barbarian 2/Ranger 3/Horizon Walker 2/Bear Warrior 10.

Hexblade

This class from Complete Warrior is not great. Its curse is Charisma-based, which is fairly awkward for you (could try to be intimidating, I guess), and what it does isn’t all that great. But it is full-BAB, has a good Will save which is useful to you, and the arcane resistance and mettle class features are decent. But what really makes this worth even considering is Player’s Handbook II, which offers one variant on it that makes it useful to you: the dark companion.

The dark companion replaces the familiar, and provides a 3×3 square area of −2 to AC and −2 to saves. It’s just an illusion, and it’s painfully vulnerable to dispel magic, but those penalties are pretty significant. It can move independently of you, and has pretty big range.

Effectively, it allows you to serve two roles for your party: locking one enemy down with your grapple, while making another enemy vulnerable to attacks and spells.

That takes four levels, barbarian takes two. The last level should be fighter for feats or ranger for skills.

Ex-Knight

This is a weird option, but I kind of like the story it tells: a knight who lost his civility and nobility, becoming savage and warmongering. Sounds neat, anyway. You start as a knight from Player’s Handbook II, take it for three levels to get bulwark of defense, and then change alignment, forsake knight, and go to barbarian. You lose the knight’s challenge features, but you maintain the Mounted Combat bonus feat (though you’ll probably never use it), shield block +1 (which you’ll definitely never use), and bulwark of defense (which you definitely will use).

Bulwark of defense makes you “sticky”—people next to you will have a hard time becoming not next to you. Less useful since presumably you’ll be grabbing them, but your grappling won’t exactly be stellar so this provides a decent back-up for things you can’t grapple.

Three levels of knight and two levels of barbarian still leaves two levels. Ranger remains probably the best option, just for the skill points (Favored Power Attack maybe if you can pick a foe you really will see all the time), though fighter could work for feats. A fourth level of knight gets you armor mastery (medium), which is OK if you want to go with mithral full-plate as your armor.