A big difficulty is that between the scientific discover, and the application years or even decades can occur. Look at how supra conductor have been known for 100 years and still have very few real life usage.
My thoughts tough
Life on Mars. I don’t talk about Martian, but if we find remain of bacteria it would be a major breakthrough in biology
-physics beyond the standard model at LHC, no impacts for commoner, but would really help physics to understand our universe
More on technology/applied science
Next generation cures against cancer with high efficiency/specificity. I think about targeted alpha therapy and immunotherapy. If these get real, a cancer isn’t 6-12 month of painful treatment for followed by a year of recovery, but something a single injection can cure.
Male birth control, would give men a better control of their own fertility and give one more option to couple where the woman can’t use birth control
high temperature supra conductors, by high temperature I mean anything above 100K in normal pressure, high current, high magnetic field. Would allow to use way more super conducting magnet than today. Imagine a world where quenching a MRI doesn’t turn off the machine for 3 months.