Particularly he had a lot of experience with the rogues which heavily inspired the original Diablo. Because of this he was initially opposed to making the game real-time as opposed to the traditional turn based, enemies only move when you do rogue style.
However as he tells the story he programmed real time movement in a few hours on a Friday night just to see what it would be like. When he tried it out for the first time and clicked the mouse; almost magically the warrior walked over and smashed a skeleton into a pile of bones.
He was convinced right then that real-time was the right choice.
That moment may have been the birth of the ARPG genre.
Gauntlet (1985) had real-time combat, and predated Diablo by at least a decade. If that isn't RPGish enough, Ultima Underworld came four years before Diablo.
Diablo definitely perfected the ARPG, though.
The first graphical game I played that leveraged the need for a lightsource similar to Moria. Not sure if you also needed food for your team.
Like a mix of rouge-likes and isometric/3d real-time with a pinch of "eye of the beholder".
If you are interested you really should check the postmortem from the GDC a few years back https://www.youtube.com/watch?v=VscdPA6sUkc
In his own words: https://www.youtube.com/watch?v=VscdPA6sUkc
https://en.wikipedia.org/wiki/Ultima_Underworld:_The_Stygian...
From my experience programmers influence the way a product looks like, they are not merely "engine creators". They have too kill some "features", create new ones that designers didn't think of.
Programming is not a simple assembly line, where designers create the whole product and workers do it as they are told.