Much as the Medivh trailer suggested, Gul'dan will be arriving in the Nexus (presumably next) to mess things up with the other heroes.
We don't have big mechanical details on the new heroes being added, but it appears Gul'dan will have a lot of shadow-based attacks, including what appears to be a "fear" style ability that sends enemies fleeing. He definitely looks like a ranged assassin or specialist, both of which would fit with his theme.
Gul'dan is delightfully colorful for a modern video game character - the color scheme seems based on his WCI&II appearances, though you can have his more muted brown robes as his secondary color scheme option. His master skin ditches the hood in favor of a big collar/mantle. His premium skin is basically the tier 12 Warlock set with its spider theme.
Next up is Auriel. It's high time we get another Diablo Angel. In terms of abilities, I think she's a melee character of some sort. She appears to have some snares, using her big ribbon things to attack foes. Just given her general theme, I would have guessed she was a support character, but the abilities we've seen - albeit briefly - suggest one of the other roles.
Auriel's master skin involves a mask over the usual black void that Angels have instead of faces, and her premium skin is a Demonic variant (which actually is a bit disappointing - Tyrael already has one of those. We should get a Naaru-themed angel!)
Additionally, there are new skins coming for existing characters:
Johanna is getting a Spellbreaker skin, making her look like one of the elite anti-magic Blood Elf fighters from Warcraft.
Arthas is getting the Mystic Kingdoms skin, which gives him a sort of Chinese wuxia look and replaces his icy attacks with jade (and also makes him look alive with a black beard - curious what his Army of the Dead will look like with this skin.)
Li-Ming is getting the Striker skin, which makes her look like a volleyball player. Her orb is replaced with a ball and her wand is now a trophy, and she's wearing comfortable gym clothing. Yes, Disintegrate becomes a rapid-fire burst of volleyballs.
Finally, there are some new mounts, like a matching horse for Mystic Kingdoms Arthas, an alternate Cyber Wolf, and a Hellsteed.
As great as Arthas is (he's my favorite Warcraft villain,) I wish some other heroes would get new skins. It's great that newer heroes like Johanna and Li-Ming are getting some love. Really I just want a new one for Xul.