I posted this quite a while ago in the Minecraft Suggestions subreddit, where it got a lot of positive support, so let's try it out in the real subreddit.

With iron and gold being the only two metals in the game, and iron being the only one with a door, I wondered what a golden door would do. I came to the following conclusion:

The golden door would be a door crafted in the way you would expect, with 6 gold ingots in a tall rectangle.

When placed down, the golden door would act identically to a wooden door. It can be opened and closed by hand by the player, villagers, and broken by some zombies.

However, if you apply redstone power to this door, it will stay locked in the position it is in (if you power a closed door, it stays closed, and if you power an open door, it stays open). It acts in this form as an iron door; you can't open it by hand, villagers ignore it, and zombies cannot break it.

Removing the redstone power from the door will cause it to revert to its "wooden" behaviour.

This is similar to doors in real life: when you use a key on a locked door, it opens (becomes unpowered) and you can open/close it as you will. If you use the key again, it locks (becomes powered) and won't budge until you unlock it again.

There could also be a golden trapdoor, which behaves in this way as well.

Finally, /u/samasaurus6 has put together a command block contraption simulating this door, to give you a better idea: https://www.youtube.com/watch?v=uS_6l7-Trus

What do you think?