The first to be open world? I know diablo 2 was a mix between open world and labyrinthic, but it has never been corridor like or hand hold (like immortal), unless you mean that the transition between places is seamless and without a single way in and out, which would now be truly different from other diablo games
And customizable characters? Diablo has always been customizable, skill trees and specs have always been a stapple, its has that rpg characteristic after all, unless you meant that the character you choose has facial and body configurations
I feel this whole written thing is scripted by someone on blizzard and people in this webpage have never played a Diablo game maybe beyond immor(t)al, barely read about the other games and try to project their ideas of the series in a comparative way, a blind leading the blind
Click bait topic, not worth the reading i gave it