As you may know, the movie that became DHWAV was originally a spec script for a standalone movie called Simon Says. Writer Jonathan Hensleigh originally envisaged the Zeus character as a Korean grocer, who would have been played by the late Brandon Lee.
At one stage, Simon Says nearly became Lethal Weapon IV, before the Die Hard franchise acquired it (the originally-planned boat-set DH3 having been nixed by Under Siege). The story originally called for the hero cop to have done something mean or unpleasant in his youth to the Simon character, who was now seeking revenge. It was felt that audiences wouldn't like the idea of McClane having done something like this in his past, then someone came up with the bright idea of making Simon the brother of Hans Gruber, thus tying it in with the original movie.
IIRC also, at one stage Lawrence Fishburne, post-Oscar nomination for playing Ike Turner was to play the Zeus character, before Pulp Fiction launched big Sam's career into overdrive. Willis' line about 'smoking cigarettes and watching Captain Kangaroo' is a reference to the song he sings along to in that movie. Also, apparently producers wanted David Thewlis for the villain (he arguably looks more like Rickman than Jeremy Irons does), after seeing him in the acclaimed Naked, but Willis felt that he was too little known. Irons, who had apparently been looking for this type of role, even flirting with the idea of playing the bad in Steven Seagal's On Deadly Ground, was deemed a suitable Rickman Replacement.