Appendix not coming from the factory is a big one. A slightly better immune system that can get through the blood/brain barrier and kill rabies/amoebas, and other brain infections like TB (without shredding brain tissue) too. If a virus like rabies gets past that barrier, you are 1 0 0 % B O N E D as your immune system is largely locked out.
Wider uterus/vagina too. Human babies have massive heads that don’t fit through too well. It’s like an orange on a toothpick. Making a smaller baby could be a possibility, but survivability would potentially go down.
For very tall people: a larger and stronger heart organ. A second one acting like a smaller turbo would also help. Heh. Failover cluster hearts. You’d probably have to have stronger veins and tissues as a result though.