The additional classes and quest have to be unlocked by creating a file containing some keywords. They decided to hide them because Blizzard only gave them permission for one additional class and they didn't like that quest.

The Barbarian and Bard are techincally unfinished. They can still be accessed, but I don't remember how.

Found it. Create command.txt in the hellfire directory and add the following line:

cowquest;theoquest;bardtest;multitest;barbariantest;

Yup, that sounds familiar, thanks.

No multiplayer though, seems we might need a modified dll for that.

Update: With a modified dll I've been able to get multiplayer to appear on the menu, but after typing in a name I get "Unable to create character."

Just use "Hellfire Enhanced", replace orignal files with the ones provided by this mod. It's Single Player anyway, and i doubt there will be any type of fixes or further patches.

https://www.moddb.com/mods/hellfire-enhanced/downloads

[Edit]

Hmm there is really something else at play, no matter if i replace every single file from the "hellfire" folder with original CD Hellfire files it will still not allow to create new character in MP... very weird, maybe Blizzard allowed Hellfire with the condition that they somehow force MP to not be enabled. I even tried with the Hellfire Ehnanced files and still the same, there is something funky going on with that specific hellfire inside the GOG Diablo folder.

[Edit]

Yes, confirmed. It seems GOG or Blizzard completely blocked any form of modification to the "hellfire" folder. I tried to rename the original GOG hellfire folder to "hellfire_bak" and then put my fully working hellfire folder that i been using to play with buddies in MP, then went in game and i could not start or create MP Character. As long as the modded hellfire is outside GOG folder Structure it will work. Not much of a problem really, just mod the hellfire folder and then place it anywhere outside GOG folder.

Don't forget

CRYPTART; NESTART;

I can't check for myself right now, but it's still possible to run the original, unmodified and completely untouched Diablo, right?

Yes, when you launch Diablo on GOG the option to play Hellfire will be there. It basically runs on a sub folder separated from Diablo.

Just what I wanted to hear, cheers.

Any chance you're still around and able to explain how you got this working? I've tried a lot of hacks in a lot of configurations at this point lol. I can get as far as having multiplayer show up, pick a class, and name a character, then I get the "Unable to create character" message in a little red window. Funny enough the first time I tried to name a Warrior Aidan it gave me some text about not being able to use reserved names after closing the window, but that only happened once. It seems worth noting that to get this far I have always experienced the enter key not working on class select. I can arrow > enter through menus until I get to class select then I have to actually click "OK" to move forward and get the red window in multi, or actually start the game in single. Seems this might have something to do with a sprint mod. Honestly at this point I don't even know what all of these mods have done, which probably doesn't help :( I have a Hellfire disc hiding somewhere I might be able to find. Could that help me?

Have you tried with DevilutionX? It supports Hellfire for online play as well.
cowquest, theoquest,nestart,barbarian and bard can be enabled from the settings menu (there is no such thing as cryptart)

the issue is related to how multiplayer saves work. multiplayer, unlike single player, saves to a system folder. i solved it in a few minuttes myself. you just need to run hellfire as administrator.