skip to 29:30 for the Q&A
John De Lancie, the voice actor for Discord, appeared at the panel of Toronto Comic-con to answers some questions from the audience. One member from the audience ask if he is going to continue voicing for animations. He replied with "I've actually done another My Little Pony, which is coming out."
So far we have no idea whether Discord is returning or a new character is being introduced. I'll keep you updated when more info pops up.