But a katana is already a masterwork bastard sword...

Depends on the system

I've seen it go many ways, but the most standard way is a long sword by a different name. No more, no less.

In D&D 3.5 a katana is explicitly just a masterwork bastard sword.

Remember that in 3.5 weapon names are... weird. What are referred to in-game as bastard swords are really closer to traditional longswords and 3.5 longswords are more like bastard swords.

Fair enough. 3.5 isn't a system I know well.

In Pathfinder Katanas are great. slightly less damage then a bastard sword for a better crit range. More bang for your murdering buck

Wait, so katanas are greatswords?

Longsword: 1d8, 19-20
Katana: 1d8, 18-20
Bastard Sword: 1d10, 19-20
Greatsword: 2d6, 19-20

Note that a greatsword has to be used 2hd

Also, katanas can be used two handed as a martial weapon, which is not a bad deal if you happen to find one without being a samurai

Wait, so katanas aren't great?

What we have here is a case of WotC material, Oriental Adventures, vs campaign setting rules, assuming Corey has Rokugan in his possession. OA is where it states Katanas are Masterwork Bastard Swords. However that is not the case in the setting book. It leaves out bastards entirely, only mentioning that Katanas are masterwork. Both books are 3.0 books and OA is what the Complete splatbooks referenced in terms of classes and equipment.

That's it. I'm sick of all this "Masterwork Bastard Sword" bullshit that's going on in the d20 system right now. Katanas deserve much better than that. Much, much better than that.
I should know what I'm talking about. I myself commissioned a genuine katana in Japan for 2,400,000 Yen (that's about $20,000) and have been practicing with it for almost 2 years now. I can even cut slabs of solid steel with my katana.

Japanese smiths spend years working on a single katana and fold it up to a million times to produce the finest blades known to mankind.

Katanas are thrice as sharp as European swords and thrice as hard for that matter too. Anything a longsword can cut through, a katana can cut through better. I'm pretty sure a katana could easily bisect a knight wearing full plate with a simple vertical slash.

Ever wonder why medieval Europe never bothered conquering Japan? That's right, they were too scared to fight the disciplined Samurai and their katanas of destruction. Even in World War II, American soldiers targeted the men with the katanas first because their killing power was feared and respected.

So what am I saying? Katanas are simply the best sword that the world has ever seen, and thus, require better stats in the d20 system. Here is the stat block I propose for Katanas:

(One-Handed Exotic Weapon)
1d12 Damage
19-20 x4 Crit
+2 to hit and damage
Counts as Masterwork

(Two-Handed Exotic Weapon)
2d10 Damage
17-20 x4 Crit
+5 to hit and damage
Counts as Masterwork

Now that seems a lot more representative of the cutting power of Katanas in real life, don't you think?

tl;dr = Katanas need to do more damage in d20, see my new stat block.

Heh.
Reminds me of this meme: http://www.kobudovenlo.nl/images/HumorImages/CutThroughTank.jpg

Remember in the series there is a grading system for the swords seen, and in the beginning, aside from Wado Ichimonji, Zoro only used basic katanas; this may very well be the way to show that 'in game' as it were