A simple construction similar to a gear assembly requiring a mechanism that can be attached to other items (bridges, doors, levers, pressure plates, etc) using two additional mechanisms (as normal).
The relay itself would transfer mechanical inputs as if it were not there. i.e., you could attach a floodgate to a relay, then a lever to the same relay, and the effect would be the same as if you had attached the lever directly to the floodgate.
The purpose would be two-fold. First, a relay could be manually disengaged, thereby preventing input transfer without fully disconnecting the the items attached to it. Second, the relay could be attached to things which will be otherwise inaccessible after construction is complete, such as floodgates that are going to be submerged, or perpetual repeaters, which would allow changing what they are attached to without physical access to the device.
You could use a single relay to link multiple devices and have them act as one. By attaching a room full of retractable spikes to a single relay, you only need to replace one connection if you want to change the control from a lever to a pressure plate/differently colored lever/etc.
Relays could also be attached to other relays to transfer control from one pre-built group of items to another in a different part of the fortress all from one 'machine room'.
Also beneficial, add a way to attach something to a relay to disengage it remotely. It would have to be a different type of connection than a normal attachment, as those would pass through (Attach To vs Attach Through?).