See Frischauer's remarks, Himmler. Hagen notes in his book that Heydrich
'once developed the theory that the posts of Fuhrer and Reich Chancellor should be separated, by which he meant that the Fuhrer should be relegated to the titular role of a President of the Reich. The Reich Chancellor, on the other hand, would be the man with the real power in his hands and this was the job he fancied for himself. But Heydrich was no mere dreamer. He did not merely toy with such ideas, but calculatingly chose his objective and worked for it with a systematic planning worthy of the General Staff itself.'
See also Schellenberg, Labyrinth. Elsewhere Schellenberg describes the cross-examination to which Himmler subjected him a few days after Heydrich's death. The Reichsfuhrer SS asked,
'Did you persuade Heydrich that he was once the only man to be considered as a successor to the Fuhrer? Heydrich himself had given me to understand this, though only fragmentarily.'