Being disappointed with First Class
(the characters, especially Xavier, were too undeveloped), I'm not excited by expanding the scope and cast (even with some being older versions of the same characters) even more. Time travel seems a bit too fantastical compared to the tone of the first two films and while The Last Stand
was disappointing, depowering Rogue and Magneto and killing Xavier were some of the better elements I'd rather not have ignored or reset. Better to further develop the First Class
characters in the past (eventually adding Cyclops, Jean Grey and Storm) or continue with Iceman, Colossus, Angel, Kitty Pryde (and maybe Jubilee with Storm, Beast and Logan as advisers) in the future.
If they really want to do DoFP and be close to the comics, Beast would be best as the character inhabitting his past body (Hoult and Lawrence were the best parts of FC).