Milk doors. Small doors usually adjacent to side entrances, where the dairymen would leave products.

A bar in the home. They are *wildly* impractical unless you are entertaining (aka giving out free alcohol) a few days a week, at which point you're just throwing away your money.

Central vacuum.

I always thought there would be a clog in the pipe inside of a wall somewhere which would render the whole machine useless. I never had one but I had friends who did.

Interestingly, though, I'm seeing these videos on instagram now showing people using them and all the comments are like the people just discovered fire. "WOW!! What a great idea!! No more lugging a vacuum around. Brilliant!!".

Bright pink or turquoise or green tile bathrooms with matching tub and toilet and sink. Whole house attic fans that could suck all of the heat out of the house in minutes.