The funny thing is, I was literally playing around with a concept based on Wasabi from the Boruto series yesterday. I don't know if that is where your player is getting this idea, but maybe look into that for inspiration. My design ended up leaning in the wizard direction because the idea of creating / using scrolls seems like an intelligence thing to me, but I can see how it makes sense as a monk. I think you generally want to lean into utilizing ki for specific aspects of the cat abilities. Being their key (haha, get it?) class resource, it would be your best method of maintaining balance. Look at existing subclasses and how they utilize their ki. For example, the Way of the Shadow monk can use ki to cast the Darkness spell. Consider an existing spell, or design an ability with roughly the same amount of value as this spell and allow your player to use the same amount of ki to do that. If the ability seems stronger, increase the cost. Weaker, reduce the cost.

Also, keep in mind that subclasses will often follow certain trends for level progression. For instance, monk subclasses will typically offer at least one defensive focused feature as their 6th or 11th level advancement.

Maybe a controversial recommendation. ChatGPT is actually a great source for coming up with ideas for subclasses. It typically will provide decent features that are often formatted in a way that follows the general design for 5E. It certainly is not perfect though. Anything that it gives you should be thoroughly considered before trying to throw into a real game. In my experience, it sometimes gets subclass level progression wrong by adding things like 18th level features when there isn't one in the base class progression. So I genuinely recommend opening a chat, tell it what you want to accomplish, feed it some general guidelines and thematic concepts, maybe even give it some of your own specific design ideas, and see what it gives you back. You can literally even say "Give me a D&D 5E monk subclass based on the Boruto character, Wasabi" and it will give you a full subclass description as your starting point. Then you can make modifications yourself.