A line editor. A clone of the venerable ed(1).
Purely for the aesthetics of the name.
And also something about zero-cost abstractions and memory safety, if you're into that sort of thing.
Ken Thompson wrote the original ed for UNIX in 1971, which would inspire
ex, vi, and most recently vim, an editor I use every day.
Ed has been the butt of many jokes, but it remains an important part of UNIX history, being an archetype for many scriptable tools to come after it.
It's simplicity (in both implementation and usage), combined with its important role in the history of UNIX, makes it an fun target for a small project.