All look like interesting concepts to toy with!

• Text editor
• 2D game - Space Invaders
• Compiler - Tiny BASIC
• Mini operating system
• Spreadsheet (hard!)
• Video game console emulator (hard!)
• Ray tracer
• Key-value store web API
• Web browser
• Stock trading bot

Also see Challenging algorithms and data structures every programmer should try.

This article talked about three data structures for efficient text editing (a.k.a. text buffer data structure): Rope, Gap Buffer, and Piece Table. It's first time I learned about Piece Table, it works like this:

• Store the text in an append-only fashion in two buffers known as original file and add file
• Store the information about segments of the files as a list of reference to the slices into the two buffers (known as "piece table")

The inserting/deleting in this data structure would simply need to breaking up items in the piece table. It seems like a good candidate as a basis to implement CRDT in a collaborative editor.

From my preliminary understanding of the nature of this data structure, it doesn't seem necessary to have two buffers, a single append-only buffer preloaded with the file should be sufficient.

UPDATE: I found this post on vscode blog extremely helpful: https://code.visualstudio.com/blogs/2018/03/23/text-buffer-reimplementation