My only comment on that list is you have two very different sets of ideas in one group. I would recommend picking one path and sticking to it.
On one hand you have 3d shape creation and collision detection, which both deal with the geometry aspects of Processing (plus they would be work well as plugins), and on the other you have everything else which is internal and deal with the rendering aspects of Processing (which would most likely have to change the inner workings of the processing rendering engine).
I would suggest picking one or the other.
If you go with geometry consider basic shapes, and/or model loading, and if you are looking at collision detection, consider perhaps even a primitive physics engine. With things like level of detail in model display, and various clipping/optimization routines for allowing processing to deal with numerous instances of objects, you'll have plenty to do.
If you go with rendering, things like stencil buffer, shading, bump mapping, and especially reflections/refractions are probably enough on your plate as it is, but you could flesh this out with some solid demos of these features in action (I can imagine a nice visual demo that demonstrates things like realtime shadows, motion blur, reflections, and so on---all which are realistic projects if you understand the core concepts necessary here).
As far as what you'd learn, in the first case you'll have a hands on look at how to work with real scenes and objects, and how professional games and programs deal with 3D. The plugin you create will have real application in a wide range of 3d projects. I personally believe proecssing has the potential for some nice game development, and this would certainly help there.
In the second case you'd really get into the internals of how a 3d rendering engine works, and how these different effects are actually implemented in software (if you made a simulated pixel/vertex shader engine, you would also get a good view on programmable shaders work). While such improvements are more for visual eye candy than anything else, I'm sure the processing crowd would be happy to have access to these kinds of tools.
So I guess it's a question of which you like working with more, polygons or pixels?
Marcello