This idea is most likely completely unrealistic, however, it would be a very powerful tool.

When you place a a hollow fuelselage, you could make it so it can make a cut from another piece of fuelselage. Let's say you have a 2x2x2 cube, and you want a circular hole straight through the middle, as of right now, you could spend hours making it look right, but if you could take a hollow fuelselage and have it run through the middle, cutting, or making any piece of fuelselage that intersects with the interior of the hollow fuelselage invisible, you could make holes significantly easier.
Other uses include making bomb bays. Being able to place the hollow fuelselage and select a "cut away" Option on the part edit menu could make a cut the size you want perfectly in your plane.
The use case that made me think of this was a VTOL system similar to the F-35. Making a circular cut out straight through the middle of the plane would allow you to place those lift fans visible in the middle of your plane.
I'm aware the idea of a bomb bays fuelselage is a popular one among the community, but this would be a simpler option, with more versatility.
Thank you for reading.