I didn’t have the time to dig through bella so i can’t give good feedback but
i really like the layered system approach over the classical udershader is a really nice idea
and if it’s flexible enough to adapt to different requirement and give a
bit of artistic freedom, it could become one of the strongest parts of your renderer.
In my opinion, what we need the most is the ability to handle complex material interaction,
skin is a good example, it’s really hard to handle complex skin shading (like blood on the skin, ink for tattoo etc)…
Also, be able to layer several different bsdf could be really nice too.
Other features like efficient motion blur, volume is also very important.
I’ll try to give more interesting feedback as soon as possible. And sorry for my english :D!
One nice addition to the gallery could be a render of the moana island!