Mario's father, famed Nintendo designer Shigeru Miyamoto, recently revealed their original intention: Mario would have fired guns.
Sitting down and discussing interesting facts about the development of the original Super Mario Bros, Miyamoto shed a little light on the thought processes on what we may consider simple standards in Mario games, like the ability to throw fireballs and chomp on mushrooms.
As translated by 1up, we find out that the original concept came from experience in Excitebike and Kung-Fu, with a need for an athletic man to leap over and around obstacles.
We felt strongly about how we were the first to come up with that genre, and it was a goal of ours to keep pushing it
The team had Mario shooting bullets with one button, and even considered including a shoot-em-up stage where players would sit atop a cloud shooting enemies. It started off as "A" to shoot, "B" to dash and up to jump.
Eventually Nintendo settled on fireballs, and made it so that "A" would be used to jump, and "B" to dash - to avoid players having the ability to continually run and shoot at the same time.
The idea behind Mario's famous mushrooms came as a mixture of folk tales (e.g. Alice in Wonderland), where mushrooms were central to growth and also prototypes being too close to Mario. When Nintendo zoomed out, Miyamoto didn't want his hero to shrink, so they devised the mushroom concept to keep Mario in the spotlight.
That, in turn, led to us calling the in-game world the 'Mushroom Kingdom' and the rest of the basic plot setup sprung from there.
Miyamoto also admits that some of the glitches encountered, like the infinite 1up trick, were intentional.