Heydrich
If Himmler dies in 1938, this will have a noticable effect on the history of the Reich, especially the SS.
By 1938, Reinhard 'Hangman' Heydrich was pretty much the #2 guy in the SS. He was also in charge of the SD, Gestapo and KRIPO by this time. His only potential rival would have been Kurt Daluege. With the amount of 'dirt' Heydrich had on everyone in Germany and in the Nazi party, there is very little doubt, in my opinion, that he would have succeeded Himmler. That being said, he was a pretty arrogant person by all accounts. This might have hurt his working relationship with Hitler. Then again, he was a pretty smooth operator.
Had Heydrich succeeded Himmler (and presumably not been killed in 1942), would Europe have seen a swifter Holocaust implemented? Would the Waffen SS been expanded more or less? His growing power, especially after his ascencion to Chief of the SS would have made the top dogs quite nervous. I'd say this would make for an excellent ATL.
Cheers,