This may seem like a really weird question but I am curious. Let me explain what I mean.
So, say you’ve made a game in a completely different engine. Is there any way to somehow have that game appear in a passage in your Twine game and be fully playable?
A more practical implementation might be: A character in your Twine game challenges the player to a game of Tic-tac-toe. Rather than code tic tac toe into twine, you embed a tic-tac-toe game you made elsewhere into the passage. After the player plays it, they continue the story.
I assume you could do the same in Twine, although it sounds like it might be more easily done in Sugarcube than Harlowe.
Another option - you could link the player to a game hosted in another engine online, and they have to play it to get a password (or some other diegetic detail) to type in to continue the game in Harlowe.
That’s what I did in The Cursèd Pickle of Shireton: there’s an Inform 7 implementation of a section of the game, and I basically linked the player (from a choice narrative in AXMA) to a completely different private online-playable parser game on itch.io. (There was no necessary code or plot relevant thing in the side game besides a preview of the endgame but it was just a lore-bit.)
Sure, Twine just creates a web page, so you can do whatever. There are certainly people using Bitsy in Twine, for instance.
Building on what @Hanon said, you can embed any web-based game by using an iframe element in certain Twine engines, particularly Sugarcube. I think you could even embed an itch.io game that supports the site’s web player.
From there you could reveal a password at the end of the game as he suggested.
There are a few forum posts utilising p5js library within twine, it is indeed possible.
Using the iframe method won’t allow you to pass variables from the in-game to you twine game. Thus after you finish the tic tac Toe game, you won’t be returning to your story. However it can be done if instead of an iframe - the end of the tic tac toe game takes the player to another html from where your game continues, such a method would however be rather inefficient.