dark knights should get a katana or sephiroth's sword

[KoroushSensei]

it is a two hand sword and right now most look like a giant triangle

[AnaviAnael]

Or they could give katanas to Samurai.

[MomomiMomi]

Please no. Just give Samurai.

[Dualblade]

Dark Knights get a good bit of influence from Cloud and Guts, and katanas aren't the first to come to mind for them.

They ARE, however, for the Samurai job. Which most likely will come out next expansion.

[MagiusNecros]

Dark Knights get a good bit of influence from Cloud and Guts

Dark Knight has little to do with Guts outside of the Darkside of the WoL being similar to the Hellhound.

[MilesSaintboroguh]

Dark Knights use greatswords. Katanas are a completely different weapon, so no.

[Maaizr]

Absolutely not

[AriaEnia]

Probably a big katana like the one this guy uses

[Sagittarian]

Probably a big katana like the one this guy uses

OP, I would recommend you get Balmung, the weapon bought with Rowena's tokens for doing the Extreme Primal weekly quest.

It's essentially what you're looking for.

Balmung pic for reference:

[MomomiMomi]

Just saying... that's not at all a katana. Also probably one of my favorite DRK swords.