What is a rare and highly profitable skill to learn as a 25 y.o person today?

Hi, I (25M) am a full time factory engineer (5 years), my only source of income. I do actual hands-on engineering work like: 1) building machines, and 2) Problem solving a faulty production unit.

I am carefully searching for a new skill to invest in during my weekends and after work. A rare & high-value skill. One that with time and honing, could someday generate a secondary (or potential replace my primary) source of income.

I have long term views. So not interested in short term "hype" skills.

What do you suggest i learn?

Thanks!