I completely agree. Plus the amount of rocket fuel that would saved if you used propelled type vehicles to fly at a certain altitude then switch to rocket fuel, similar with jet engine vehicles or could use the sabre engine. It all depends of thrust and mass.
The E.M drive is still being researched by a few countries. Like they did for the ion drive, they could increase the thrust so it will be useful. I agree, Squad should introduce metallic hydrogen tanks, metallic hydrogen engines, cryogenic tanks and cryogenic engines, if they have not been added already.
Yes, I know about the Far Future Propulsion mod. But I would like some, if not all, of those propulsions, added into the stock parts, the propulsions that are being used or researched, not hypothetical propulsions, maybe later.