Wearable computers were going to be a thing several times - look at the progression of Prof. Steve Mann's wearables[1], around the late 1990s when the Twiddler chording keyboard he's using became available and the hardware was small and light enough to be more like a big walkie talkie than a freakshow. Again with Timex Datalink watches, and things like PDAs and Palm Treos, again with Google Glass, again with early smartwatches. They were basically all wiped out by smartphones. Maybe smartwatches are in fashion again now, but maybe not.
Netbooks - the idea of a low power device which can hardly do anything. From the Asus EEE to the WebOS tablet to the One Laptop Per Child to the Chromebook. You could say the Chromebook did become the next big thing, but it's kinda been trumped by the Macbook M1 which has smallness, battery life and none of the web-only restrictions, and competing on price was always a struggle between the low-end Windows laptops and the high prices of decent Chromebook hardware. On the low-price end, Raspberry Pi came out.
Airships, they were going to be the great military air power of the WWI era but too many crashed, they were going to be the long-distance passenger and freight vehicles of the pre-WWII era, but the Hindenburg and other accidents happened, they have been going to be reinvented by Airlander (2001), Techsphere (2003), DARPA / Aeroscraft /Lockheed Martin (2009), similar work by Google Loon (2011 - 2021), Airlander with Graphene (2016), Airship do Brasil (2017), Buoyant.aero (YC 2019?), SkyCat, among others. Their intersection of cost to build and certify, lifting power or freight capacity, weather dependence, size, safety, profitable use cases despite low speed, is quite small.
Segway.
[1] https://cdn57.androidauthority.net/wp-content/uploads/2013/0...
