Reiner:
Yes, I would love to help this great project. Just note that my skills are limited to graphic design.
I don’t want to step on any toes though, so we (and muller) must find some common ground on what the road ahead will be.
About Apache: I know Cycles is licensed as Apache 2.0 and can be used in any software both commercial and otherwise.
Davis: Thanks!
I think most of the existing icons could be changed to more simple, single color icons quite easily, if they are all available as vector files?
In my opinion consistency is key when it comes to icon design. Some icons may be transferable. But linewidth, curves etc will be different on many 
Draise: Thanks!