Output node in the node editor. Similar to Arnold, so you can connect things like the dirt node to the material output to see it on your object alone to tweak it. Can be difficult to get the settings of some nodes right without seeing them alone. Currently piping it into the diffuse of materials to tweak it, but would be quite helpful if there was a way to solo a node to output.
Random value node that can change aspects of a material at random. Example, I'm currently using a gradient to control the mix value of two materials, the first material is a glow and the second material is a glossy. This makes the tip of my object glow as it grows on. There's a seed value in the gradient, but I have no means of controlling it via nodes or otherwise, so every material I apply it to is exactly the same. As such, I've had to duplicate the mix material a dozen times and adjust the seed value. Would be quite useful if there was a means of applying randomness to materials via a node. I understand this may not be possible.
Additional gradient controls like those found in the Cinema4D gradient, likewise more noise options for Octane noise like the standard cinema noise. For the life of me I couldn't figure out a way to animate and loop octane noise at a reasonable speed. Cinema noise allows you to set a loop interval which would be an amazing feature in Octane noise.