An item akin to the sliding door, except with the height of a wooden gate - 3 blocks high, maybe even 2. Preferably coming in two variants - black sliding gate and light sliding gate. An object like this would be perfect for futuristic gardens and water handling mechanisms.
In fact, about any small metallic blockade would do. It could even be a small 1v1 cube, a wiring object.
sounds like a good idea for a mod. you might suggest it over on the modding forum if you don't have the time/skills to do it yourself .