I like the idea of optional inputs.
I find the idea of using variables for them okay, but not 100% straight forward.
Grasshopper has it on some nodes that if you zoom in deeper, you can activate additional inputs.
Usine, a node based audio application has three interface states for nodes: "Collapsed" (no inputs visible, all wires go to one visual port, node is as small as possible), "Used Inputs" (only the most important inputs and those actually in use are visible) and "All" (all available inputs are visible). So you can noodle away with all inputs visible and then collapse to one of the two other states later.
I could also imagine that you could "activate" a field as input in the inspector GUI.

Cheers,

Tom