Since I’m not much of a lore guy in Diablo I’m simply curious about couple things. This is in regards to upcoming (“soon”) expansion.
So the first question is… are guns/gun powder a thing in Diablo universe ? Why not making that kind of a new class ? Since they already made Monk 2.0 in Vessel of Hatred. Why not making DH 2.0 but with guns ?
And 2nd question is… how come in every ARPG we always play the same class over and over and over and over again ? Now, imagine playing an actual Demon ? I mean… why TF not ? Why not having some fun for a change ? I don’t need to play “a good guy” all the time. There’s an option to play a Sith in Star Wars games. Why not a Demon here ? This could work for multiple reason. One… finaly something new unlike Monk 2.0 or copy-paste Paladin class. Two… as a Demon we could actualy fight to bring Diablo back. And there you go… Diablo in a Diablo game. See how easy that would be ?
Just a thought 
Cause the game wants to keep the medieval setting, I suppose, adding guns is sort of like crossing a line into a new setting. And trust me, I’m a big fan of that setting but I don’t think it would feel very Diablo-like. Picture the first game. That’s the root of it. Swords and magic.
As for your second question, you really don’t play a good guy. You play a guy who does not want to die. And yeah, while I’m a bit annoyed by the dialogue the player chracters have, I wouldn’t say it makes them good or evil, it just makes them a guy or gal with a cause - to keep the world from ending. I mean, you can see that the Diablo world is messed up. Heaven is not good, not for the humans of Sanctuary, the people who fight demons aren’t good either as you saw from Vessel of Hatred with the knights going from protectors to oppressors.
And while your idea might be interesting, it’s just not compatible with the game as it is right now. Even if you get to play a demon, your goal will also be to prevent the destruction of Sanctuary. So in my opinion, it would be a waste.
As for your question, why people want to play the same classes, it’s simple, it’s the same reason they keep playing Diablo games, because they enjoy them.