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

Definitely not the first to support local multiplayer. Unless you don't count D2 having Lan as an option without needing a battle.net account and if you also don't count the D3 couch co-op.