It's hard to list 15 because I think certain groups of disease have the same root cause, so I'll list the diseases I think need the most attention in terms of root cause:
1.Developmental disorders- in my future practice I hope to focus a lot on things that negatively impact development. I think this is the #1 aspect of health that is being hurt by modern society in terms of lack of education. Toxic chemicals from canned food, plastic toys, water bottles, hospital equipment, etc are having a huge effect on the development of our kids. I think many people who have had mysterious illnesses all their lives or had one that was triggered once they reached a certain age could have them explained by something that hurt them during their development either directly or by the lack of something. For example, now about 1 in every 3 women have a C section birth, and a CDC report found that only about 1 in 3 women were breastfeeding after 3 months. The implications of these numbers are huge. It means that all the babies born by C section and who aren't breastfed aren't getting the good bacteria they need to populate their gut and thus protect them from infections in the future, allow their immune system to recognize the difference between good bacteria and everything else and tolerate the good bacteria, and even allow their brain to develop properly in many cases. So I think if a baby has a mom who ate healthy, had low stress while pregnant, and avoided all these things, it's much more likely he or she won't have many of the other diseases I list below (of course it depends on the environment they grow up in and the diet/lifestyle they have as well).
2.Digestive disorders (leaky gut, dysbiosis, crohn's, UC, etc)- the root cause of most diseases imo is either located in the gut or started out there and spread to other areas. If you have a properly functioning gut with the proper balance of good and bad bacteria it's hard to have a chronic illness of any kind.
3.autoimmune disorders (food allergies, thyroid, MS, etc)
4.neurological disorders (depression, anxiety, ocd, adhd, autism, insomnia, etc)
5.metabolic disorders (obesity, diabetes, etc)
6.imbalanced hormones (thyroid, chronically elevated cortisol, estrogen dominance, etc)