GUI plugins

**What do you want to see?**
The ability to create plugins for the GUI

**How would it help players?**
Making custom mods for the GUI

**Alternatives**
N/A

**Screenshots and mockups**
N/A

**Things to consider**
How to even implement this? What would a GUI plugin do? Would it be part of a mod or separate from mods altogether?

**Related issues**
N/A