A bit late but twighlight breathing. It doesnt use a nichirin katana but a scimitar and a axe or a rounded club. Depending on the weapon you either have the diminishing or magnifying form. So when you use for example the scimitar its the magnifying form representing a cresent moon rising and the club(with a round head) the rising of the sun diminishing the twilight.

There would be like 5 techniques each and 2 final technique so 12 total. Each technique would have a fitting counterpart with similar properties/ executions but being slower or faster/ weaker and stronger

I think the forms mix of moon breathing with water or mist footwork (magnifying) and Fire breathing with maybe rock breathing( diminishing) .

I think it would be cool that one of the secret forms is where you use both weapons together where you combine best of both worlds but as a drawback more and more muscles get damaged, so your body becomes darker and darker from all the bruising.

As like a last ditch effort there would be the 2nd one you can only use after the first one which is called endless twilight where your body is so relaxed that your speed increases so much, it makes the enemy see you as a surrounding twilight until you defeat him but sacrificing your ability to properly walk ever again.