
    The multirender for layered transparency. It just does a brain-dead sequential render to prevent the worst artifacts when transparent objects occlude other transparent objects: https://github.com/3dmol/3Dmol.js/issues/224 I found this very simple to implement
    For me, lights customization is important, especially lights that have a built-in and user-controllable fade to replace fog: this is pretty invasive though
    Color schemes with additional properties
    Embedded capturers for video and image including with transparent background

Things that are complicated and/or not very useful:

    Shadows (complicated, quality issues without even more complicated solutions like CSMs, which I haven't attempted yet)
    Fixed animations (rarely have a use case for these)

Things that I like a lot for fun but probably don't generalize as much:

    New materials, also live (animated) ones (that take time as a shader variable)
    Option to add a render step at the very end to apply image effects (primarily developed for a fake depth-of-field): the 2D shaders are quite simple to write and the extra texture+rerender is tolerable

