Well, it comes down to this. A katana is about three feet long and does not require special training to use one-handed. You can call it an acorn squash for all I care.
What I am trying to say is, "A katana should function mechanically as a one-handed weapon with an improved critical range that may also be used two-handed. Having it function as an exotic weapon when used one-handed makes no sense. It makes it too difficult to wield, and creates very strange situations when you start equipping NPCs in Japanese-inspired settings."
EDIT: And it's not necessarily that D&D used longsword "wrongly" except from the standpoint of late medieval fencing scholars. It's more that it's hard to come up with a term for a normal sword (which is exactly what Basic D&D called it). "Arming sword, not especially large" doesn't exactly trip off the tongue.