Like a "Smart Wagon", that maybe allows you to build combinators on the train itself? If I can put constant-combinators on a train (and if 0.14 allows train contents to be "read into" a train station through the circuit network), then it would allow some very fancy stuff to be done.
Ghost-building of wires between two points would be more than sufficient. An A* Pathfinding across electrical poles between two points is all that would be needed.Continuous wire building while running, somehow
All I really want though, are for "Set Filters" (for Stack Filter Inserters and filter Inserters) to actually be sane with regards to operation. For example, if Filter Inserters took the five largest signals in the network as their filters (and if stack-filter inserters took the single-largest signal), then these would be much, much more useful with the circuit network. As it is right now, "Set Filters" mode is hard to work with. There's no real way to control priority of the filters on the signal... I could build a sorting network, but that requires a good chunk of work.