In a campaign about yokai and other spirits ghost touch is a useful weapon affix. But since there are no such thing as an incorporeal undead (technically) incorporeal undead do exist in D&D. Here is a 5e version of ghost touch that works around this.
A ghost touch weapon deals damage normally against incorporeal creatures, regardless of its bonus. The weapon ignores an incorporeal creature’s damage resistances and immunities. The weapon can be picked up and moved by an incorporeal creature at any time. A manifesting ghost can wield the weapon against corporeal foes. Essentially, a ghost touch weapon counts as both corporeal or incorporeal.
What is an incorporeal creature?
In 5e it is any creature with the incorporeal movement trait is considered an incorporeal creature to satisfy the ghost touch requirement.