As a car enthusiast, I can think of a good one, the Ford Nucleon.
During the 1950s and 1960s, there was considerable interest in nuclear power and its potential applications. This led to the idea of using nuclear energy to propel cars. The concept behind a nuclear car was to utilize a small nuclear reactor to generate steam, which would then power the vehicle's engine.
Of course back in those days, this was extremely futurustic and some at the time thought this would be a game changer, but ultimately, the safety aspect was one of the biggest reasons why this idea was dropped, and I probably don't have to explain why it may not have considered to be safe, I mean, it was using nuclear power, so even if the engineers tried to make it as safe as possible, IF something went wrong, it would have been catastrophic.
Ever since then, the interests in the automotive sector has shifted to Electric and Hydrogen.
Still, a very intriguing concept car and idea.
Outside cars, you have blimps, and I personally believe if we tried to make something like a hindenburg today with existing technology, we might have been a lot more successful than back then (as it goes way back to 1930s), there are still some blimps used occasionally, I also don't believe those use hydrogen(?), but they are not the "game changer in air travel" it was once seen as, although we can't rule out a comeback.
What about you guys?
The Metaverse, I guess? It's funny how living in a virtual world has been this hyped-up concept for decades and it finally comes out and it's just kind of...lame for lack of a better word. Maybe it's too early to tell, but it feels like the Web 3.0 Metaverse push hasn't lived up to the hype.
Aside from that, I'd say the Xbox Kinect. Maybe it's just me, but I remember that when the Kinect came out there was a lot of hype about how it was going to revolutionize how people played games. But I don't think we ever really got a Kinect game that lived up to that hype. To be fair, I remember a lot of articles of people doing interesting things with Kinect it's just that none of them really had anything to do with gaming.
Did actually think they would pull something off? It was all 100% advertising hype for a knock off VR Chat.