...e mid-60's - "Mad Man"-style Manhattan! This would serve as a bridge between the 1940's Captain America and the 2000 Iron Man. Also, how cool would it be for Howard Stark brainstorm with Reed Richards, with a young Henry Pym happily taking notes? You also get to have outsized adventures with a fictional past-Marvel history. If you MUST have a crossover, Dr Doom's time machine will suffice as the movie's DeLorian!