>I don't want to create an interface that is super generic and has lots of possible uses if 95% of its functionality stays unused.

Yes, that's what I fear, too

>Most likely I'd want a sentry gun or guard tower that works on its own, without me connecting it to a switch and switching it on first. Same thing for doors

I think so, too. Similar to your factory example

That's why I always try to push the actual application to the foreground: I would propose designing a hand full of gameplay elements (FOR PLAYERS!) that work well together and are usable ingame without an additional interface and enhance the gameplay.
E.g. think of situations in maps where you would use feature X and that feature would use a lever and a door.. blabla

The interface would then be designed to solve these actual ingame applications.
Also see Newtons reply to Mimmo's first post. A lot of theoretical applications are probably not of any use in any map we currently have. And I think the generic, core gameplay elements should work on a majority of our existing maps.
I think a big part of why we don't have these things yet is because we didn't really find any suitable applications. Even though I loved the idea when we discussed it back in the days (2011 or so) with PeterW.