Now that the Advance Class Guide is out, the answer has changed (as @Lord_Gareth suggested it would in his January 2014 answer).
And now that the ACG errata is out, the answer has changed again. Changes incorporated below.
As of today, there are two published feats (three if you include Mythic) that expand the set of finesse weapons:
1. Dervish Dance
(which requires a 13 DEX, Weapon Finesse, 2 ranks in Dance, and proficiency in the scimitar) lets you count a scimitar as a finesse weapon (and add DEX instead of STR to damage)...as long as it's sized for you, used one-handed, and you aren't "carrying a weapon or shield in your off hand."
2. Slashing Grace
(which requires a 13 DEX, Weapon Finesse, and Weapon Focus) lets you use DEX (instead of STR) for damage with any one handed or light slashing weapon one has Weapon Focus in. As with Dervish Dance the weapon has to be sized for you. Additionally (since the errata)
You do not gain this benefit while fighting with two weapons or using flurry of blows, or any time another hand is otherwise occupied.
Note though that it doesn't allow one to use DEX to hit with a one-handed slashing weapon unless one has at least 1 level in swashbuckler. Swashbuckler's get the Swashbuckler's Finesse ability which says:
a swashbuckler gains the benefits of the Weapon Finesse feat with light or one-handed piercing melee weapons.
That combined with this text from Slashing Grace:
When wielding your chosen weapon one-handed, you can treat it as a one-handed piercing melee weapon for all feats and class abilities that require such a weapon
lets swashbucklers (but not others with Weapon Finesse) use DEX to hit with a longsword or similar weapon. (A fighter with Weapon Finesse and Slashing Grace gets DEX to hit with her rapier but still uses STR for damage. If she switches to a longsword she gets DEX to damage but is back to using STR to hit.)
Also note (as per the original question) that Slashing Grace makes a bastard sword a finesse weapon for swashbucklers...but only while wielded one-handed.
Bucklers and Swordsmaster's Flair are OK!
The following FAQ was asked about Slashing Grace but applies to Dervish Dance as well.
What exactly does it mean that “You do not gain this benefit while fighting with two weapons or using flurry of blows, or any time another hand is otherwise occupied?” Can I use a shield? What about a buckler? Can I use flurry of blows? Brawler’s flurry? Two-weapon fighting? Spell combat? Attack with natural weapons? What if I throw the weapon? What about swordmaster’s flair?
Slashing Grace does not allow most shields, but bucklers work because they don’t occupy the hand. Flurry of blows, brawler’s flurry, two-weapon fighting, and spell combat all don’t work with Slashing Grace. Attacking with natural weapons beyond the weapon you chose for Slashing Grace also does not work. Slashing Grace only works with melee attacks, not thrown attacks with a melee weapon. Swordmaster’s flair should have a sentence added to it that says “Carrying a swordmaster’s flair counts as having that hand free for the purpose of abilities that require a free hand, though you still can’t hold another object in that hand.”
Why No Love For Rapiers?
As an aside, Jason Bulmahn (Lead Designer) has said that DEX to damage was a late addition to Slashing Grace, and the design team didn't realize that it made a longsword significantly better for a swashbuckler than the more iconic rapier. To solve that, they've announced that the following feat will be released in the upcoming Pathfinder Player Companion: Advanced Class Guide Origins:
FENCING GRACE (COMBAT)
Your extreme style and fluid rapier forms allow you to use agility rather than brute force to fell your foes.
Prerequisites: Dex 13, Weapon Finesse, Weapon Focus (rapier).
Benefit: When wielding a rapier one-handed, you can add your Dexterity modifier instead of your Strength modifier to that weapon’s damage. The rapier must be one appropriate for your size.
In addition, if you have the panache class feature, you gain a +2 bonus to CMD against attempts to disarm you of your rapier while you have at least 1 panache point.
Mythic note: one more way to get DEX to Damage
While the core of this question is about adding to the set of finesse weapons, both Slashing Grace and Dervish Dance also add DEX to damage (arguably their bigger benefit). For completeness, I'll add one more DEX to damage option: Mythic Weapon Finesse.
If you happen to be playing with the optional Mythic rules, this is generally the best solution. The Mythic Weapon Finesse feat's only requirement is that you already have the general Weapon Finesse feat. It states,
When using Weapon Finesse, you may also use your Dexterity modifier instead of your Strength modifier on your damage rolls.
So (unlike the options above, you can use any shield, flurry of blows, all of your natural attacks, etc.). It even tacks on the further benefit that > If you carry a shield, its armor check penalty doesn't apply to either the attack rolls or the damage rolls.
Yes.
The soulknife ability clearly states that if a soulknife has Powerful build the knife it manifests is larger to match the ability. So you would deal increased damage due to having a larger knife than say.. a regular human.
By design this also means if you can find a way to get Enlarge Person to increase your size, then you would also benefit from the soulknife damage increase granted by powerful build as well.
Form Soul blade states:
A soulknife must choose the form of her mind blade at 1st level. She can either form it into a light weapon, a one-handed weapon, or a two-handed weapon. Once chosen, her mind blade stays in this form every time the soulknife forms her mind blade. The light weapon deals 1d6 points of damage, the one-handed weapon deals 1d8 points of damage, and the two-handed weapon deals 2d6 points of damage. All damages are based on a Medium-sized creature wielding Medium-sized weapons.
This constrains the Soul blade to a certain type of weapon, such as longsword, shortsword, trident, etc. Whichever you choose when you first manifest it. The size changes based on your own size, which may have variable scaling.
Best Answer
No, Versatile Design does not let you finesse a lance
So the word “light” is being reused here to mean two different things, and it’s kind of a chicken-and-the-egg problem.
Light as a “handedness,” so “light weapons” as opposed to “one-handed weapons” and “two-handed weapons.” Light weapons can be used with Weapon Finesse, take smaller penalties when using two-weapon fighting, cannot benefit from Power Attack, and so on.
Light as an English-language adjective in the term “light blades,” the name of a fighter weapon group. In this context, a “light blades” refers to any of the weapons found in the light blades group.1 Not all weapons in the fighter's Light Blades group are finesse-able (scizore, chakram, manople), and simply being in the group doesn’t make them light weapons.
In other words, the light blades were chosen to be in that group because they are light—but they were already light before the fighter got involved. Adding them to the fighter weapon group “light blades” didn’t make them light, they already were.
Likewise, if you add the lance to this group using Versatile Design, that doesn’t make it a light weapon. A light blade, yes, and things that care about what fighter weapon group you are using will recognize that, but it doesn’t otherwise change how the lance works. It will not allow you to use Weapon Finesse with the lance.
However, effortless lace may let you finesse a lance
Effortless lace is a very useful magic item that can allow you to finesse weapons you ordinarily couldn’t, along with other useful properties. Specifically,
(Effortless lace item description)
Notice that this can only be used on a one-handed piercing or slashing weapon—a lance is clearly piercing, so that’s no trouble, but it’s a two-handed weapon. However, note also that
(Lance weapon description)
This does not, strictly speaking, make the lance a one-handed weapon. The rules as written would not allow you to use effortless lace on a lance.
Your GM, however, might. I would. I would allow you to use the effortless lace on the lance, but it only benefits from the lace while you are mounted.