Something occurred to me this morning. Within the past couple of weeks, we've learned the details of two of the Doctor's regenerations: from the Eighth Doctor (Paul McGann) into the War Doctor (John Hurt), and from the War Doctor into the Ninth Doctor (Christopher Eccleston). And yet both of those regenerations were the result of "causes of death" that we've seen before. Eight was killed by a spaceship crash, just as Six's regeneration in "Time and the Rani" was triggered by the crash of the TARDIS. And the War Doctor regenerated due to old age just like the First Doctor; he even quoted Hartnell's line that his body was "wearing a bit thin."
And the last regeneration we saw before that, David Tennant into Matt Smith, was due to radiation poisoning -- the same thing that "killed" the Third Doctor in "Planet of the Spiders." Which means that every regeneration we've seen in the past several years has been due to a cause we've seen before. He hasn't had an "original" cause of death since "The Parting of the Ways." (And even that, his body being devastated by time vortex energy, could arguably be lumped into the radiation-damage category.) Or at least since "The Stolen Earth," which should probably be counted as at least a partial regeneration (though there's disagreement over how full a regeneration it was and whether it counts against the total). Although that was a case of the Doctor being shot, which was also part of the cause of the Seventh Doctor's regeneration (that was arguably due more to botched surgery, but I think that would fall under "complications" from the gunshot wounds).
So here are the Doctor's mechanisms of regeneration to date:
Old age/wearing thin: First Doctor, War Doctor
Enforced regeneration: Second
Radiation/energy overdose: Third, Ninth, Tenth
Fall from a height: Fourth
Ship crash: Sixth, Eighth
Gunfire or subsequent complications: Seventh, Metacrisis Tenth (oh yes, and Fake Eleventh)
(And yes, there's a big difference between a Dalek blaster and a semiautomatic rifle -- but technically a Dalek blaster is an intense dose of radiation, so either way it's gonna fall into an existing category.)
So out of the twelve (or eleven and a half?) regenerations we've seen, the causes arguably fall into only about seven categories -- which are the causes of the first seven regenerations. There hasn't been a new category added since 1996, just variations on the previous ones.
Which leads me to wonder how Matt Smith will regenerate in the upcoming Christmas special. Maybe he's due for another fall from a tall structure. After being poisoned.