Ok. So as your build already has a lot of set feats and such that you have to take we want to build upon that instead of trying anything new. So let’s take a look at feats and magic weapon enchants that effect critical hits or ways to free up any feats.
Not all of these options are possible for your exact build but I will add them for anyone in the future who wants to improve the build and has different stats than yourself.
Feats
It might be a little late as you have already taken it but the exotic weapon proficiency feat was not required unless you have a RP reason for the wakazashi. Kukri has the same crit range, they do deal less damage at a d4 though. The base weapon damage won't make up much of your damage in the long run though and it would allow you to dedicate your first feat to something more useful. You might ask your DM if he would allow you to use the downtime retraining rules located here.
Prerequisite Combat Expertise, INT 13
Depending on your team you might also consider this feat as it allows your character to give up his crit to deal normal damage but the next ally to hit with a physical attack auto crits. So your big barbarian friend can deal massive amounts of damage if he has a low crit range weapon but a high crit modifier.
Prerequisite: Human
Following the theme of using your high crit chance this feat gives you a +2 on a single attack roll, saving throw, skill check, or ability check of your choice before the end of your next turn if you crit or roll a 20 on a saving throw.
Equipment Enchantment
As you are after damage increase I will be focusing on magic weapon enchantments.
Magic Weapon page for reference.
Speed is a +3 enchant.
Speed is one of the more common enchants that players use. It grants an extra attack for each weapon it is on. So as your character dual wields that would be an additional 2 attacks at the character's highest BAB each round.
Element Burst
Again following the Crit theme the element burst enchants. Each is a +2 enchant
There are four of these types Flaming, Icy, Corrosive, and Shocking each of them auto adds 1d6 of their element to all of your attacks. And when you crit they deal an additional 1d10 damage. The cool thing about these enchants is they can stack! So you can have a Flaming Burst, Corrosive Burst weapon that deals normal damage +1d6 fire +1d6 acid on normal strikes and when you crit it becomes Normal crit damage +1d6 fire +1d6 acid +1d10 Fire +1d10 Acid.
Touch attacks are hard to get, the normal weapons that do that don't do damage (nets etc.).
It's somewhat expensive (more so than normal weapons, less than 3l33t magic), but you can use firearms (touch attack in first range increment) with the Holy Gun paladin archetype. It changes smite evil so it can be used with ranged gunshots.
You can rely on ray or other touch spells a lot; smite evil will work with them (though it'll be harder to get multiple attacks a round as you go up in level than you can with just plain attacks). Maybe pick a revelation that lets you do at-will rays.
In general don't obsess over smite, it's a nice but mild bonus. You should be more concerned about whether you are going to be a valid melee or ranged combatant or not in general.
Best Answer
A question like this really depends on party make up.
If you're a primary damage dealer, you should go STR, unless you fight primarily big baddies who you can Smite without hesitation (then go CHA).
If you're aiming more towards a tanky person, you should go CON for the HP.
If your party is hurting for heals, then you really want to go CHA for more Lay on Hands.
Based on your description of playing a castigator-type person, I would assume you want to go damage, so I would say STR is probably your best all around choice, CHA if you want more specialized damage, but also more utility in spells/smites/lay on hands/saves.