At the moment, the Katana seems quite worthless, for PvP and PvE alike.
So my suggestions is, to quicken it’s heavy attacks for abot 25-50% and double the damageoutput for the dash-attack.
Reasons:
The heavy attacks are so slow, the only effect is to make you an easy, defenseless target. Plus, the are obsolete, because you do 3 light attacks in the same time. This might be ok in efficiency for pvp, but is utterly boring.
The dash attack has to be timed absolute perfect to have any effect at all or you waste stamina, defense and time for “no” damage at all…
A bonus should be a real bonus for perfect timing (sorry for pvp, if it seems to strong, but “I didn’t see it coming!”, is no good excuse for dying from an attack that needs at least 3second to launch…).
Your best bet at getting the Katana ‘buffed’ is to go after the weapons that outperform it.
So… nerf everything?
Other than the dash attack to reposition yourself it doesn’t really have much going for it.
Seems to be the way players want it. Give it to them.
I wholeheartedly support this proposal, @Talyna .
I love the katana., It’s one of the few agility weapons, takes skill to master and needs more love after the heavy nerf it suffered. Make the katana a more viable weapon, Funcom! 
This wouldn’t need to happen had they not needlessly nerfed it in the first place.
They need to restore the damage from the dash attack. And while at it they need to return bleed and poison timers to their previous durations.
Disagree on that, low tier Javelins are pretty good melee weapons at the start and higher tier ones make decent budget endgame ones.
I’d argue they’re complimentary arms however, the Katana is (or should be) a ‘main’ weapon.
The problem is… it’s been reworked, buffed and nerfed more than any other weapon at this point (feel free to correct me, i’m not sure).
For something that basically got it’s own mini-patch you’d think they’d have some idea of what this weapon should be capable of doing in the game.
Your interest is purely pve, so i will test this weapon to see it’s possibilities as they are right now. Yesterday a friend of mine send me a build he uses even in pvp
So i will test it and let you know if it needs buff or what it may need “more”. Maybe i will figure even a challenge with this weapon. These walk throughs i do, give me a good amount of time with specific weapons, numerous opponents and help me understand better and use better a weapon! I will let you know the soonest possible!
So, first test was done
I didn’t use blade of the seven wings but a normal star metal katana with master kit.
The rest is the known!
Heavy attaks, slow, but not slower than hammer, 2h axe…
Light attacks, very good.
I don’t know the possibilities of this weapon, but as i see it, it can stand well enough in the game. I will have to think for a challenge that will push me to learn better (i need motivation).
My dear @Talyna help me, i beg you, think of something difficult and let me know, i really want to figure out what katana is and how far you can go with it!
Arena Champion? 
Arena champion is “the teacher” boss. This particular boss exists on the game (at least for me) to give you a really important message for this game.
Fighting alone is better!
I may have no deep knowledge with katana, but she have died from me with all the weapons in the past.
Arena champion is giving you a full combo chance before she attacks! It’s one of the less bosses in this game that you have full combo chance!
I have to be honest my friend @Azazane, after 3.0 i never visited her to see if what i just said is changed, so i will pay a visit! Thank you for your beautiful suggestion! Still i would wish a full session, to start from absolute 0 and face specific bosses only by playing Katana, so any story line or suggestion would be really appreciated!
I hope the Javelin gets some love. Despite their many issues they are one of my favorite things to PvP with.
Honestly anything to mix up the spear, dagger, war hammer meta. It’s been neat seeing the great axe make some nice showings with the AGI pen perk.
Edit:
The katana can still light spam ok. I think the daggers just out do it in the AGI category without the dash. I wouldn’t hate seeing something change with it to give back some of the identity that it lost with the nerf.
Oh wow… moar katana discussions 
It used to be really good for crowd control, it still kinda is since on agility your other alternative is the short sword that has a very narrow hitbox. I think you should go and to the Yuggoth event at Scavenger’s Berth with nothing but a katana, that is currently one of the hardest PvE content on EL, especially if you don’t beat around the bush and pull the ones from the back as they only ever seem to come 4-6 at a time.
I would’ve been happy with even a compromise.
Here’s the thing, the katana was too strong indeed, but not because of a few PvP players getting randomly killed because of it, but because of it’s sustained DPS potential if people min-maxed for it.
If you max both Str and Agi to get all the finisher bonuses and small bonuses from the STR tree as well, put on some skelos master armor with both consumable buff and have a high damage katana - essentially min-maxing for that. While at the same time focusing on trying to execute just the finisher dash as fast as possible… the katana could get up to just shy of 1k DPS sustained even over several thousand damage points
Typically though if people don’t min-max it was still hovering around 400.
After discovering that, I also changed my tune and said that maybe it needed some change even for PvE…
To put that in perspective a max damage voidforge gladius with the special stabby combo can barely keep up to 400 and you run out of stamina much sooner and most other weapons when used very very well hover around 200-230.
However I still strongly disagree with what they actually did, which was making it the same as the dash without combos.
The combo system is still in place though just like it was… and that supports damage multipliers…
So in order to buff the weapon so that it does a bit more damage after doing a dash following 3 other hits, all they’d have to do is edit a number… and I think even PvP players would be happy with that version since it would arrive in a single hit and thus countered by perks and armor.
Oh well 
anyway I do have a mod that reverses the change completely should anyone want the old dash back in their single player game
No, No, Noooooooooooooooo .
No nerfs to other weapons or any weapons. Make weapons better not make weapons worse. If a weapon is shown to not be effective then work on it until it is . This Nerf need is self defeating and just makes fights (pve or pvp) more stale over time and predictable especially pvp.
I think you misunderstood me here, and I think I was definitely unclear…
“Worthless” is the wrong word. UNINTERESTING would have been better.
So, I want the Katana to be fun again. Not a “light, light,light attack” weapon till the opponent falls…
I already use daggers most of the time. For fun. Indeed the Katana is still apotent, but boring weapon…
I’m on your side in this. Spear could be fun, if more diversity (as melee weapon) was given to it.
This…! 
You still can archive quite a lot with the Katana. It will not be easy though. But I fear you will be bored to death… 
And this…!
The katana should be improved exactly because of pvp. You don’t need to improve anything for pve, as a skilled player can beat all content with any weapon.
What I would like to see was the katana becoming a good agility alternative weapon to the spear, a strenght weapon, on pvp. This could be interesting, because for pve… pfff. Pve is easy. We all know that.
The Portal to Yuggoth, as @Xevyr pointed out, is probably the most difficult content right now, and I practically soloed it (because my unleveled thrall died fast) with a legendary sword I got from a chest. I can’t get it to spawn now, however, or any other event from chapter 1. Blowback could be a hard nut to crack, but all the buildings make it easy. There’s not much dificulty left on pve, so let’s focus on where the katana mostly lost its efficacy: pvp.
As it stands, the katana is not a serious contender for pvp. It can still be effective if you fight against a much less skilled opponent, but if you find a guy who knows his trade and is good with a spear, you’re toast.