Self-riding bicycles. The dream.
Let me explain. They're not really like self-driving cars (they could be, but this isn't what interests me), because I don't want them to transport me anywhere. Once I'm on the bike, I want to ride it.
The real purpose (as I'm imagining it) is for the bike to be able to transport itself when I'm not on it. Why? For many reasons! Mainly so I can cycle whenever and wherever I want without having to accept that to cycle somewhere, I'll have to cycle back, or if I want to cycle back from somewhere, I'd have to cycle there.
Bikes without humans are very light, and could move very quickly with only a small motor - you could even charge the motor by pedalling when you're on it.
So many magical possibilities! You can cycle somewhere and can have a few drinks if you want. You can carry stuff into work and cycle home, or vice versa. If you fall on your bike far from home, you can get a taxi back (most of which can't fit bikes) and send the bike back by itself.
Remember in Pokémon how Ash could whip his bike out of his pocket like some sort of bike demon? That could be you! A bunch of rider-less bikes zipping along the streets, like loyal dogs returning to their owners.
Obviously there are a multitude of small practicalities to solve, but the hardest is to make the bike be able to "drive" by itself. Most other problems are solvable, with the massive caveat that they may make the whole thing too expensive.
Anyway, that's the dream. If I end up working for Google or something on this project, I won't complain too much.