I'mma repost a comment I just recently put up on the comments board for the Characters page.
"Alright, you guys. Decided to revise my list of all the possible guest stars for FiM. Again, if you've got any in mind, please add to the list.
Johnny Yong Bosch, Yuri Lowenthal, Hynden Walsh, Steven Blum, Jeff Bennett, Candi Milo, Charlie Adler, Dee Bradley Baker, Janice Kawaye, Lauren Tom, Grey DeLisle, Wendee Lee, Susan Blu, David Hayter, Clancy Brown, Tom Kenny, James Arnold Taylor, John DiMaggio, Kari Wahlgren, Nancy Cartwright, Tress Macneille, Rob Paulsen, Kirk Thornton, Cree Summer, Kath Soucie, D.C. Douglas, Jennifer Hale, Scott Menville, Jim Cummings, Carlos Alazraqui, Roger Craig Smith, Russi Taylor, Tom Kane, Cathal J. Todd, Frank Welker, Wally Wingert, Hedy Buress, Gwendoline Yeo, Fred Tatasciore, Kevin Michael Richardson, Greg Cipes, Stephanie Morgenstern, Scott McCord, GACKT, Quinton Flynn, Phil LaMarr, Hayley Joel Osment, Patrick Seitz, Elizabeth G. Daily, Cathy Cavadini, Christine Cavanaugh, Troy Baker, Laura Bailey, Nolan North, Jim Ward, Mark Hamill, J.G. Quintel, Kyle Hebert, Paul Eiding, Cam Clarke, Kim Mai Guest, The Dobson Brothers (Paul, Michael and Brian), Rino Romano, Josh Keaton, Roger Jackson, Patrick Seitz, Cristina Valenzuela (Cristina Vee), Jameison Price, Doug Erholtz, Vincent Martella."
Again, you guys let me know if there's a voice actor you'd like to hear in the upcoming episodes.