A while back after skype got rid of their sdk I was annoyed that the app didn't unfurl gifs. So I reverse engineered their api for sending/updating messages and I realized you could update a message that contained a file. The only problem was the file had to be on their servers or a client wouldn't display it. So I reverse engineered the ios app as well to find a way to get a file on their servers (I don't recall why I couldn't do it from the web api). After that I had two things: a way to get files on their servers, and an api to update messages. So I took each frame of a gif, uploaded to their servers, and then used the api to update the message at the rate of the gif.

Of course there will be people making discord bots out there laughing at how simple this sounds, but the combination of reverse engineering as well as applying that to affect a new feature in the app I thought was pretty novel.