I'm sick of loads of regenerations being bug plot twists and instead of dying in action, he sacrifices himself for a companion or something.
No I want something big, and I want the doctor in pain!
Absolute agony. Him crying out in pain. If we went with my cyberman idea (see page 1), then maybe they capture the doctor, start converting him into a cyberman by injecting him with some liquid with hurts him, or even start operating (we don't know how the very first cybermen converted, maybe they've learned to do it more humanely.) and then River or someone saves him, takes him to the TARDIS and he regenerates.