It would be amazing to see a PLC that could combine a number of the existing logic components into a program (Stormworks has a very good example of one in-game). This would make introducing complex logic into subs far less fiddly than the current system of daisy-chaining a dozen components together.

+1, It would be much easier to connect signals to a PLC, even if its a limited number, than trying to lay out all of the individual logic gates.

I think a miniaturized circuit system would be more useful. Basically, an item that allows you to create circuits, which is then later compiled into game code and inserted into submarines. This would allow for much faster and more compact execution of circuits, as well as enabling sub creators to lock away an entire complex circuit, while still remaining configurable in game. Unlike a microcontroller system, the developers would not have to make or integrate a programming language, while still keeping it sandboxed and simplified. With a circuit board, it still would need to be sandboxed, but it would be easier to implement and wouldn't require much modification to existing ships if they want to use it.