I think if you had Claude Fable build a lightweight terminal like Ghostty, you'd get a good one.

I doubt that very much.

An LLM may be fine to implement quickly an application that more or less works, but I have not seen any evidence that any of them is able to write well optimized code, whenever the application is not just a duplicate of something older.

Ghosstty and a few other newer terminal emulators are not only lightweight, but they also implement very fast GPU rendering, which is one of their important advantages.

So not only their set of features matters, but also the achieved performance.

Of course, one could use an LLM to write a very fast terminal emulator, but for that a competent programmer must be included in the feedback loop, which would require many iterations and running benchmarks and emulation conformity tests. It cannot be done by vibe coding with a prompt like "write a lightweight terminal emulator".

Here's one such project: https://github.com/Dicklesworthstone/frankenterm

However, it is more focused on multiplexing.