It would be great to find a cure or treatment for all disease. But that is not one invention but hundreds.
Although I don’t work in this area, I think we need a packaging material that can be used for everything, including food, liquids and fragile products. That material needs to biodegradable and recyclable. I can’t stand landfill waste and always feel so guilty when I make the problem worse.
There are many good engineers already working on making devices more affordable. I would like to join them and invent a way to make heart devices more affordable so that poorer and developing countries can afford them too. That would mean engineering materials that are cheaper but have the right properties (like stretchiness or toughness) and don’t harm the body.
A machine that made everybody more rational! If people thought more rationally, we’d be much better equipped to address the world’s problems as a species.
Outside surgical application, but within the realms of something more viable and within reach, I’d love to invent super thermal insulating material that was low cost and that could be easily fabricated into old homes and new around the world. Making buildings more energy efficient would dramatically help to reduce pollution and help to prevent climate change (although it’s a bit late now!).