The idea I had was that there would be new node-types that are similar to node groups - the iteration rule would be a group with the current iteration as a single input, and the result of the iteration rule as an output. The Current iteration would be a group, too, taking the iteration and any constant parameters as an input. Then the combination rule would also be a group that is optional, for combining the results of various iterations. I think this UI might be approachable in any node-tree, but of course it still needs some work.
I wasn’t trying to go off topic-- of course this isn’t the place to talk about Cycles! I just wanted to offer a UI suggestion, although it looks like you’ve already put quite some thought into this! Thanks 
As for this comment:
I would love to see the ability to have node-trees interact with unlike types of node-trees.
I was just thinking of a far away future, after Modelling and Particle Nodes are already implemented.
Hey @jacqueslucke I have some questions regarding the particle roadmap ⚓ T73324 Particles Nodes
I can’t find anything related to instancing objects to particle positions/rotations, except at milestone 2
(Use cases are supported (particle scattering, weathering, …).)
Will we have instancing of objects at the earliest at milestone 2, or will basic instancing be implemented in milestone 1 already.
I’m wondering because I think the Functions system is already pretty capable to be used for a lot of motion graphics stuff, we would normally do in animation nodes. When it is not to much work to implement, groups for manipulating instances could be build and tested already.
So when milestone 2 would be active, there could be already a lot of group nodes ready for shipping with blender or at least some tests could be made in advance. Personally I would love to rebuild the feature set of c4d mograph effectors and it’s cloners, also to learn how these systems work 
Anyway, do you have already plans or technical details on how such instancing could be implemented in a good performant way? Will there be a new type of system which draws the instances nested, or will they be shown in the outliner, like it is with animation nodes at the moment? (Which would be not that good of a performance I quess.)
Please ignore if you already answered this questions ^^
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Pretty late to the party here, but I would like to add that Houdini has a “For each” node. Actually there are 6 of them:
-For Each Connected Piece
-For Each Named Primitive
-For Each Number
-For Each Point
-For Each Primitive
-For Loop with Feedback
It creates a beginning and and end node, with a special backdrop for the stuff in between. So that is one loop method.
Another is simply putting down a Wrangle node where you can just type your own vexcode and make your own loops manually.
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Having a sort of language like vex one could use along with nodes would be very nice indeed… And I find the loop interface very well thought, it’s completely clear what it does and where it starts and ends. All very good examples.
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