So i was looking at 2 handed swords the other day and saw both of these swords followed by seeing a  verison of the Muramasa which did interest me. i thought maybe at first it could just be put in the slot NPC but with the description of both saying the following
An Oriental Sword Named after a Legendary Japanese swordsmith and An oriental sword that is the masterpiece of an ancient Japanese smith this got me thinking maybe a way to upgrade both of these
Masamune does not have an slot version but with the penalties it enacts (67% less defense and -5 str. it does give 30 flee and 2% attack speed though) but compared to many swords nowadays its main selling point of an 200 attack stat really falls with competition such as Kry  . Death Guidence  . Violet Fear  Tao Goo Lyeon and the Heavy Blade added in OGH. Being able to weaken/remove the penaltys or the ability to add a slot or 2 may be able to keep it up with these swords.
The Quest NPC could be based in Amastu Dungeon level 2 or 3 named either the Legendary Swordsmith or Muramasa. For the first parts of the quest these items share the same steps. In order for the Swordsmith to believe you are worthy of his time you will be task to defeat an large number of Firelock Soldiers (like maybe 200?) once you have defeated these soliders he will ask that he is not satified and will ask the player to kill a small amount of Tengu and Shinobis (25 of each?) . after you have proven yourself to the smith he will discuss how he created the Muramasa and the Masamune Swords and how he would long to see his creations again. If you happen to have either sword in your inventory an option will appear to show him and he will thank the player. he will ask the player to leave him for an hour while he tries to figure out the issues with his swords
After an hour has passed and the player returns to the man he will say while he has not discovered a way to remove the curse on Muramasa he has discovered a way to unlock its potential (giving it 2 slots) but he will require some items to be able to do this. Money is one thing he will need and he will ask for around 2m for the service. he will also need a Muramasa  to work on it, and a load of other items such as Oridecon and Steel (to reandforce the blade) , Live Coal , Coal and a level 3 fire bolt scroll (to set a forge up) and Sharp Paper (for an idea of sharpness?) . of course more items could be added to this list this is just a rough idea. After all the items have been gathered you will be given a Muramasa  for your work.
Likewise you can choose to upgrade the Masamune. If the idea of a slot or 2 was to be added it would go very simular to the Muramasa slotting quest needing metals and coal and such. If the purifed idea prehaps to purifie the blade he will need the following. 2m for his services, The Masamune Sword to work on , Holy Water, Light Particles and Holy Marbles to uncurse the sword and some darkness rune and evil horns to learn more about the curses?. again this is mainly a rough idea of what sort of items it could require. after that he will give you a Masamune, or Purified Masamune whichever seems the best. Purifed Masamune stats could be simular to what it has but without the -5 str penalty and either no Def Penalty or maybe reduced to a lower rate like 25% or 33%. if slot option then it would still keep these.
Why do i think a Muramasa  would be a big thing in the game? i feel it would make for a very interesting weapon. it would serve its own purpose. being more of an auto attack crit oriented build sort of a weapon. it may have a lower attack but having both a 8% attack speed boost and a 30 crit rate could take the weapon as a mid/end game option with the right cards like we see with the other end game swords.
As for Masamune i mean even with slots or if purified it would still be a mid tier at best but it would be in a much more usable stat of either being able to put cards in to help it with its flaws or with less/ no of the flaws it could work quite well even as a non slotted weapon.