It is clearly impossible and would be dangerous to plan for everything that might afflict a human being. Fortunately, it is not necessary to attempt such planning. When planning is conceived as something that can be done over time and in relationship to your health condition, it becomes possible to create a more realistic planning experience.
When adults are healthy or do not have a progressive, life-limiting condition, there are only two important future medical decisions that are needed: Selecting a well-qualified person— typically a trusted family member or friend—to make your health decisions when you cannot; and to determine when a serious, permanent neurological injury would be so bad that you would want to change the goals of your medical care. With this approach to planning, you can express a plan to identify who will make your decisions when you cannot and when a serious brain injury would be so bad that the goals of medical treatment should change. In making this plan you can indicate what treatment you would want or not want, but the actual decision would be made in the moment by your appointed healthcare surrogate.
This plan needs to be reviewed and reconsidered over time, but you really do not have to plan for any other medical decisions until you have a serious condition that is getting worse, and/or you are suffering significant or more frequent complications. At this point in time it is clear that more complications might be expected. Here, what you need to be prepared for is how to proceed with the next complication when the outcome of that treatment is not good. Consider when an outcome would be so bad that you might change the goals of your care. Again, such planning prepares the patient or the healthcare surrogate to actually make medical decisions in the moment.