I've been working on PMBS, an open-source Paper Mario battle system, on and off for a couple of years now and just released a 1.0 version!
* Battle Menus
* Action Commands
* Items and Badges
* Dialogue System
* Move Sequences (what happens during the move)
* Status Effects
The battle can be configured via a config file and is described in more detail in the link.
While working on this, I learned more about not only two of my favorite games, but also MonoGame, and really enjoyed it. The battle system is extensible and supports all types of interactions, including some that the original games did not feature!
I started learning shaders and more about graphics in general for this project, and the community here has been really helpful. I'm not new to MonoGame, but I have never delved into its more advanced features and stuck with what I was familiar with. This project forced me to get out of my comfort zone and learn something new, and I really appreciate all the help!