Every conversation I see on 40 codices certain armies ALWAYS comes up as being on the *** end of the spectrum, in such a way that it almost doesnīt matter how good or bad the player is, unless the other players are extremely unlucky and/or the player is extremely lucky certain armies will never get in first in the tournaments.
At least they wonīt get into first place or close to unless they bulk up on allies.
Now while o have nothing against allies I donīt like the way that they need to be alpha omega for certain armies in order to win, to me allies should be flavor only, sure they can offer some fun game mechanics, but in the end your army should be able to fight and possibly win it all on its own two feet.

So letīs say GW hired you to re-write the rules of the one codex you thought was the weakest and bring it up to a point where it could actually win tournaments on its own.
And just to be clear here I am talking tournaments with no limits at all if the rules say you can do it, you can do it, so your army will have to be one that can stand up to every death star list out there and have a reasonable chance of beating it.

So which codex would you improve and how?

With me it would be Dark Eldar main because itīs what I play.

Now the Dark Eldar as I understand them is the fast scalpel, the thousand cuts army that will bleed an opponent to death rather than finishing it off instantly.
So a Dark Eldar army should have two things in my mind weapons that can slowly peel off the enemy’s armor and be the fastest army there is.
The moment a Dark Eldar army stands still they die, because in exchange for speed they have sacrificed their armor to such a degree that even a Imperial guard last gun can be deadly for them.

Hereīs what I would change with the Dark Eldar:
Now for the sake disclosure I donīt have huge experience on tournament and the system I have most experience with is Fantasy, so most suggestion on improvements are from what I have read on the internet:

While each kind unit has a fixed amount of movement I would add 1d6 to that movement, the dice roll is final and canīt be re-rolled.
The only exception should be Dark Eldar Reavers which not only get the 2d6 roll any Eldar Jetbike does, but also has the option of re-rolling one of the dices.

All Dark Eldar units would have a 5+ Feel no pain from the beginning.

All Dark Eldar units should have the Fleet special rule.

Power from Pain is the same as in the Heamonculus Coven.

Also the Dark Eldar has a special rule that means in close combat they always strike first no matter what, against other Dark Eldar players roll a d6 the one who gets the highest strikes first.
While I am no fan of removing overwatch or directly nullifying it, iīm looking at you Eldar Banshees, I do think it should be remodeled for Dark Eldar on account of them having virtually no armor.
Dark Eldar Whyces and Incubi should on account of their speed for any unit to roll a 1d6, on a 1 and 2 the unit is not allow to overwatch, on a 3 and 4 they do so but at half BS and on a 5 and 6 they fire overwatch like normal.

Mandrakes are described as being able to emerge from any shadow slice the throat of their victim before sliding back into the shadows again.
Sounds almost like what the Eldar Warp spiders are able to do, so make Mandrakes able to do the same.
Able to teleport around the board and gear them for hit and run close combat.

Change the Dark Lance abilities so it mimics the thousand cut analogy.
For each hit that doesnīt destroy the Target it give it -1 to its armor save.
It could be like this 1st hit nothing happens, 2nd hit -1 to armor save, 3rd hit -2 armor save and so on and so forth till itīs reduced to only be able to manage a save on a 6+
In fluff terms it could be described as the Dark Eldar able the hit the same spots over and over again and each time remove a little amount of the armor till almost nothing is left.

Grotesques have the same abilities as Dark Lance slightly modified 2 hits are needed before they get the same effect.
I imagine in fluff terms it could be described as the Grotesques beating at the same spot constantly till broke through was able to tear open the hull and get to the crew.

Also Raiders and Venoms should have the special rule Fleet of foot or something like that that makes it so that if the Raider or Venom is destroyed, the crew will only be wounded on a 6+
In exchange if you do hit that 6+ no saves of any kind is allowed, neither feel no pain or It will not die.