There are a lot of ideas that sound wonderful in theory but never get past the concept stage because of unforeseen problems or limitations. Here are just two that I recall from the past:
At one point–probably about 30 years ago–Nissan announced that they had developed a Freon-based propulsion system for cars, and that this system was so economical that it would eventually supplant the ICE. I don’t recall the exact details of how this system worked, but apparently it never got past the concept/press release stage, as far as I can tell.
I can recall an article in either Popular Science or Popular Mechanics (I read both as a child), back in the '60s, that illustrated and described how a massive horizontal flywheel underneath heavy trucks would essentially store energy while the truck was moving, and allow it to operate in urban conditions with much greater efficiency and with much lower fuel costs. Have you ever heard of this being actually used?