Fighting incorporeal undead just sucks. At higher levels, its not as bad due to most characters likely possessing a magical weapon by then. However, at lower levels, its frustrating to be a fighter and be fundamentally useless because that sword your carrying will simply pass through the ghost's intangible form. While 5e does lessen the frustration by only giving the ghost resistance against nonmagical weapons (meaning you'll still deal at least a little damage), I thought it'd be fun to create a different way you can handle these bastards without resorting to magic.
The idea would require a little more work from the DM, but I feel it could be pretty interesting. Most ghost stories have the titular spirits attached to something or someone, so attached that it prevents their spirit from passing on to the next life. What if a player finds someone tied to the spirit's previous life, something they cared deeply about, and that object allowed them to have power over the creature?
For example, let's say the ghost possessed a special necklace that once belonged to a very closed friend. The PC could find that object and present it to the ghost as an action in combat, forcing the creature to make a DC 15 Wisdom save. If they fail the save, the ghost is considered incapacitated. Every turn thereafter, the undead makes another Wisdom save for free, allowing them to act as normal if they succeed.
While the character isn't damaging the incorporeal undead, they at least can effect it in a significant way. Also, it creates an interesting way for characters to deal with ghosts and other incorporeal undead besides combat. The players might have to delve into the spirit's past, hunt down one of these objects (maybe even a relative), and use it against the creature.