Add-ons
I've heard a rumor that Blizzard North is planning add-on packs for Diablo2I that should be released in July and December of 1999.
It has also been said that in addition to new monsters, levels, and items, each add-on pack will also include 2 new characters!

Exclusive - Blizzard North interview at Gamestar magazine
The German magazine, Gamestar, just had a 4 page interview with two of Blizzard North's designers who shared some more info on how Diablo2 will be like, and we at www.oocities.org/TimesSquare/Ring/4266 immediately put it up online (first on the net... again!).

   There will be 5 different character classes in Diablo2. One is a genuine fighter, one is some kind of sorcerer. 2 characters will be female. One of them will be an Amazon who is very good in using bows and spears, the other will probably be a female demon.
   A new kind of weapons will be the spears which can be thrown at enemies.
   Every kind of character will have his own spells. So in order to cast a certain spell you will need to be playing a certain character. This will surely add to Diablo2's replayablity value.
   The inventory (number of boxes in inventory screen) will also depend on the character you'll be playing. For example, fighters will be able to carry more than sorcerers.
   Every part of worn equipment changes the look of the character in play. You will see a difference in your character's look if you hold a scimitar or a longsword for example.
   The cities will be populated by a lot of different characters. They will not only stand in their shop (as in the original Diablo) but walk around and "live their lives". There will be different kinds of shops, but not every shop type will be in every town, so you will have to travel often.
   There will be fights in the towns! An example would be a big monster raid, where the character would have to save the town.
   There will be more interaction with the NPC's. For example, two different characters would like you to get them the same artifact. You will need to decide to whom to give it and face the consequences.
   There will be 4 cities in the game, surrounded by a vast wilderness area.
   You will not be able to get to all 4 towns at the start of the game. There will be a quest in which you have to clear a passage through the monsters in the mountain to gain access to the next town. A lot of the quests will make you wander between the cities to fulfill them.
   You will be able to combine certain items together in order to make them more effective. For example, gold bracers will not be very powerful with leather armor, but if you have a gold helmet, gold chain mail and gold bracers, the bracers will be a lot more effective!
   The fighters will be able to learn some special attack moves. Like an all-around-sweep, which damages every monster standing around you, but leaves you more vulnerable and taking more time than a normal attack.
   The upgraded engine will probably include multi-colored lightsources and animated rivers and lava that will actually flow downstream.
   Blizzard are considering making multiplayer games capable of hosting more then 4 characters at a time.
   There will probably be something like a trading post where players can meet with other players to exchange items.
   Blizzard North has about 20 people working on Diablo2 right now, and for those of you looking forward to Warcraft Adventures, it will probably be pushed to the second quarter of 1998.
   There will be 3 types of monsters in the games:
Standard monsters - Like the skeletons in the original Diablo, but this time they will come in a lot more variations, they will wear different armor and hold different weapons.
Special monsters - Like the unique monsters with their own name in Diablo. They will almost always leave a unique item when they die. Special spellcasting monsters will use different spells then the rest of their kind.
Boss monsters - They will be really big and dangerous freaks. They will each only be in a game once. As Erich Schaefer said: "You will instantly know when you are facing a boss!!!". This boss monsters will probably always have quests built around them (Kill the certain boss, get a certain weapon from a boss, etc.)

Thanks to Jimbo from Germany for letting me know of this interview!

The brothers
This is just a rumor, but it is very possible that Diablo2 will include not only Diablo himself, but also his brothers Baal and Mephisto.

Exclusive News - Multiplayer
Diablo2 Multiplayer will bring up to 8 people together at one time.

Exclusive report from inside Blizzard's offices!
A source from Blizzard that wishes to remain anonymous has given us the following info:
   There will be a much larger and more involving plot, and each character has the chance to do different quests. Depending on how you act throughout the game, i.e. not doing a quest, your relationship with an NPC may change, so certain things in the development of the plot will be different.
   You can hire help such as mercenaries.
   Armor and weapon appearances change depending on what you equip.
   There are multiple endings to the game, which one you get to depends on your actions and the character you play.
   There will be a lot more videos in Diablo2!!!