The option of creating new particles
choose particles for blocks more than just sparkling textures (like the shiny ores)
and also options to change the slime particle color, for resourcepacks that change the slime color

I would love this! I like to change the slime color to Orange.

I too would very much like the ability to change the particle colour
I had a dig through the jar and it appears the colours are handled within the mod as apposed to the texture because my first thought was that I'd manually change it myself

There also seems to be a somewhat hardcoded list of particle effects, so using a resource pack to try and add new effects also doesn't work. I created a resource pack to change effects, and while I was able to change the texture of the particles, when I created an identical copy of the bone.json file called bone2.json, I was unable to assign it to any entity.