TL;DR
An underground entity for circuit connections, similar to belt and pipe undergrounds. Especially useful for space platforms.What?
A small 1x1 entity with red and green wire connection points. Like a power pole, it has no logic of its own, but its tooltip shows signals on the connected wires.It can connect to other entities with a decent reach, at least as long as a medium power pole. It can connect to other underground circuit conduits with a longer reach, equal to a large power pole.
Wire connections between conduits are not visible, because they're underground. But when you hover one, or hold any combinator in your cursor, you see the connections as nice thick lines, exactly like the fluid visualization.
It would ideally not be limited to horizontal and vertical connections like pipes and belt undergrounds are. But if the interactions with gaps in the space platform are too messy, it could be limited this way, and would be handled the same as other undergrounds are now.
Graphically, it would be half the height of a medium power pole. Enough to stand out from combinators, but not drastically higher. The guiding principle is visual clarity and reduction of clutter, so they should be thin and not obscure the view of entities behind them.
It can be rotated like the constant combinator can, even though they are 1x1 entities. This allows rotating it in place to easily distinguish which wires are connected to it.