Ever since Oblivion, a tradition that Elder Scrolls seems to have is to have important characters in the main story be voiced by big name actors.
In Oblivion we had Patrick Stuart, Sean Bean and Terance Stamp.
Skyrim had Christopher Plummer, Max Von Sydow, Jim Cummings, and Laura Bailey.
Online had John Cleese, Bill Nighy, Kate Beckensdale, Alfred Molina, Lynda Carter, Michael Gambon and Malcolm Mcdoyll
So, who would be good choices for ES6? Here's who I'd like to hear and the kinds of characters they could play.
Keith David. Mr David is a familier voice on TV and other games, and sometimes he even does full length movies. He's been Goliath in Gargoyles, Dr. Facilier in The Princess and the Frog, Captain Anderson in Mass Effect and countless other voice roles. With a diverse list of characters under his belt, I could see Mr. David playing any type of character, though if ES6 was set in Hammerfell, it would be logical to have most of the voices to be from dark skinned actors.
Gary Oldman. Mr Oldman has been Ignitus from The Legend of Spyro trilogy so he has had some experience in video game voice acting. Other voice roles have included Sir Ruber from Quest for Camelot and Lord Shen from Kungh Fu Panda 2. Ignitus was a mentor character, while Ruber and Shen were villains. So Mr. Oldman could be a good choice for either a wise mentor, or a major villain, perhaps even the main villain of ES6.
Yuri Lowental. Mr. Lowenthal is familier voice in Anime and Video Games alike and his resume includes Suzaku Kururugi from Code Geass and Sasuke Uchina from Naruto. So I could see him playing a major protegonist, or an anti-hero of sorts.
Steven Blum: A familer voice from anime, video games, and tv, Mr. Blum is best known for voicing villains and anti-heroes, such as Ares from God of War, Zoltum Kul from Diablo 3, and Dehaka from StarCraft 2. He might make a good Daedric Prince or creepy villain.
Tara Strong. I know what you're thinking, "What!? You're suggesting the actress who voices Princess Twilight Sparkle, Timmy Turner and Bubbles for The Elder Scrolls!?" Oh she's done more then those. She's also Ben Tennyson, Ashi, and Barbara Gorden in The Killing Joke. So she can do more serious stuff as well. She'd undoubtably be cast as a protagnist, maybe as an aspiring mage or as a warrior.
John De Lancie. Mr De Lancie has come a long way from being Q in Star Trek. He's now notable for being Alarak in StarCraft 2 and Dscord from My Little Pony Friendship is Magic. Given the similarities between Sheogorrath and Discord, I could see Mr. De Lancie voicing Sheogorrath, or as a smug arrogant villain or anti-hero.