1 - I know some things are not possible now that the engine switched to deferred, like anisotropic rendering. It’s a bit of a give and take: do you want custom shading, or the ability to render thousands of dynamic lights on the same mesh? I definitely wish there was a method to enable some forms of custom shading, though, like cel shading to work with deferred lights. You can always use the light vector method to feed into emissive, but that is a bit of a cop-out. At the moment, the stylized demo is your best bet. If you want UE4 to work for you, you need to find a method that the engine likes. It doesn’t really like translucency, and it doesn’t like crazy non-realistic rendering that isn’t at least somewhat based on the real world. If you want to make your own shader suite to be compatible with the material editor, good luck.
2 - The example you showed me looks like something that can be done with GPU particles in Cascade… unfortunately, there won’t be specular highlights unless you turn on forward rendering, and doing that will be very expensive because of the GGX. A lot of these tech demos are really awesome, but integrating it into a game might not be practical because of the world of light types and rendering features. If you use Epic’s systems, you will be amazed. I got caustics underwater from a light function, and the caustics would actually blur and light up beneath the subsurface material on my character and shadow itself between blades of underwater grass.
3 - There are some predetermined compute shaders like tessellation available for everyone… to make one yourself though, yeah, I don’t think that’s possible.
I do believe I have made the suggestion to support other models of specularity and have access to other functions, but by and large unless you’re using C++ to code a game, rendering stuff should not need to be coded. There is a UE4 solution for practically everything, except some of the more extreme cases. Unless you specifically want to program compute shaders on the GPU, it’s really not necessary. You can use GPU sprites in Cascade. You can use Parallax Occlusion Mapping within the material editor. You can’t do cel shading specifically, but if you want something like the stylized demo, you can totally do that. You can make a high-end cartoon style like Ratchet and Clank fairly easily in UE4. If all you want to do are things that UE4 can’t do, then, maybe it’s not the engine for you. But if you take advantage of the tools you have, you’ll be surprised what you can make.