Maybe you can aim for someone else. After all, we got Heydrich in OTL. Any other slimy Nazi at the top may be possible. Or hell, one of the innumerable super generals they had. Capping Rommel, anyone?
British commandoes tried to get Rommel, raiding his N African HQ, but it was one he had moved on from and the result was the death of the force's leader, the son of Roger Keyes of WW1 fame
Regarding Hitler, German conspirators had to work in the real world of a Nazi-dominated state - ie if they kill ONLY Hitler, then they end up with Goering or Himmler as Fuhrer and things could be even worse ! And they would certainly be worse for the individuals involved ! That's why July 20th ended up being such a detailed plan, involving Fellgiebel of Signals and the chief of staff of the Home Army - after Hitler's death they had to move quickly to put something else in place. It also made planning an assassination an extremely complicated business
Less political minds did try other assassinations, or stood near to Hitler with a gun and contemplated killing him (Manstein's aide de camp) but it came down to, on the one hand Hitler's paranoia and change of plans screwing things up, and on the other hand the thought of what was the point of shooting Hitler as he was only one person
British plans eventually came down to two stumbling blocks - -1- the mass revenge on the populace after the death of Heydrich, and -2- that Hitler being such a piss poor strategist made him very useful to the Allied war effort!
If they wanted to kill someone useful, then perhaps Speer or Goebbels would be a relevant target
Best Regards
Grey Wolf