I would love a way to define a particle for a given GUI - one possible idea I had was a JSON schema that maps a GUI classpath to a texture, for example:

{
  "gui": "org.violetmoon.quark.addons.oddities.client.screen.BackpackInventoryScreen",
  "texture": "quark:textures/misc/backpack_gui.png"
}

Alternatively, texture could be a particle, however I believe that would require code registration of a new particle? Which you wouldn't be able to do easily if you were a modpack developer working only with assets and data.

Additionally, a code-based API for mods would be neat too, allowing them to register custom rendering for a particle