Just another node added to the collection! 
Loop node
All you need to have is a nodegroup in your material, with an input with the name ‘iterator’, and for passing values from one step to the other, the nodegroup has to have an input and an output with the same name.
Here’s an example
Nodegroup to be used as a step in the loop:
And the node in usage with the respective result (here simple voronoi clouds)
it needs some improvements to deal with linked sockets (changing the nodegroup will remove all previous links to the node).