Medical equipment. The software for medical equipment is super arcane stuff, locked behind patents and licensing that only serves to drive the price up. For someone in a less economically powerful country the cost is increased and the quality of care is reduced massively by this licensing. If an open source version of the software for things like an MRI, CAT scanner, ultrasound, and so on could be made it would enable the updating of old hardware to keep it running longer and also make repairs much more affordable. I also think that internal defribulators would benefit enormously from being open source as under the current system they are hackable and can be exploited to wirelessly trigger a shock which can be fatal.

Also consider continuous glucose monitoring and insulin pumps. By combining those two technologies you can essentially make an artificial pancreas and make diabetes much more manageable. The hurdle here is not technological, it is legal. The thing that has slowed down the production of an artificial pancreas is software licensing and legal risk management, nothing technical. Consider this applied to the possibility of a continuous allergy monitoring device which could detect anaphylaxis and administer adrenaline, or maybe markers of a seizure approaching to trigger a much smaller dose of an anti epileptic medication. All unavailable because the development of that technology would not be feasible under the current licensing and liability system we have.

To be clear, I am not saying any of these cure the underlying condition, just that if medical software (and ideally hardware) was required to be open source more of these tools could be developed to help people live better lives independently and safely, and at a lower cost.