Inform (http://inform7.com/) is fun to play with. Understanding how the interpreter wants things formatted is a bit tricky, as the developers are trying their best to algorithmically understand English sentences, but when you get the hang of it, the system is kind of like a "plaintext Twine"

The playable interface is different (more text-adventure than CYOA), but that also gives you a lot more flexibility to experiment with ideas that aren't simple "guided tours". I think I still prefer Twine's approach for putting stories together, but I do wish there was an interpreted version so I could use my own editors and feed it a Markdown document, instead of needing to use their mouse-driven editor