As I just announced on Reddit, I've got some basic scripts for modding Rivals of Aether now that I'm calling "Wrastor". Everything appears to work, though it's admittedly not very user-friendly at the moment.
I wrote my prototype in Ruby, though it's a pretty poor language to keep the code in. In order to support less-technical users, I'd have to package it up somehow so that it doesn't involve installing Ruby. As a result, my next step is to re-write it. Since half the point of writing these modding tools is for me to mess around in my spare time, I figure I'll re-write it in the latest "hip" language: Rust. I wanted to re-write it in Crystal, but they still don't have good Windows support (which is a requirement since I'll be providing these tools to Windows users).
At some point in the near future, I plan on documenting how I came to figure out how to mod the game. Since most of this work should be useful for other games made with GameMaker Studio, it'll hopefully be useful for the wider game modding community (that I assume exists - I haven't really been working with anyone else on this).