The heroes in question are the Wizard and Necromancer characters from Diablo 3 and Diablo 2 respectively. They are Li-Ming the Wizard and Xul the Necromancer. In an interview with GameSpot, Blizzard revealed what it was that led to them choosing these two characters to be added to the game.
According to Blizzard, “Necromancer has been a character that’s been asked about for quite some time. Since 2013 people have been asking about Necromancer, and now we’ve finally got a kit that we were happy with. The idea for Li-Ming the Wizard actually started as specifically as, ‘Is there an easy-to-understand, but very high skill cap hero that we can do? An assassin, someone who can really blow people on. Who would that go on?’”
It will be interesting to see how these characters will fit in with the rest of the heroes. Blizzard has actually been on a pretty steady pace when it comes to introducing new heroes, not surprising since they want to make Heroes of the Storm equally as competitive, if not more so, than other MOBAs.