A chemistry thing for me, and on a very small scale, I’d invent a series of proteins (these are the little machines of the cell) in order to easily split water (H2O) and carbon dioxide (CO2) into their consituent elements, hydrogen gas, oxygen gas and carbon (as a solid) or as methane (CH4, the simplest hydrocarbon).
There are a number of ‘natural’ systems that do this already, particularly photosystem 2 (which happens in every plant leaf) but whilst we have been able to determine what they look like (a molecular ‘photograph’) and how they work, we are not very close to being able to replicate it ourselves. If we could it would solve the world’s electricity and gas problems overnight.
In the 30s we invented printing on paper with ink.
In the 80s we invented 3d printing of plastic objects.
In the (distant) future I can see an object printer capable of printing anyting we have a building plan of.
On a more theorical level, inventing an object to transform matter in energy and vice-versa would solve a lot of the humankind problems.