A really, really lame version of Juggernaut appeared as a henchman in The Last Stand, but since that doesn't exist anymore there's plenty of room to do him right. He's probably the iconic X-Men foe other than Magneto, after all, and ginormous ogre-like beasts that need a full team to take them down are perfect foils for a movie like this. Plus, let's face it - immobilized-genius Xavier having a nigh-unstoppable brute for a brother is just too devilishly ironic to ignore.
There is one glaring problem, though: Juggernaut, traditionally, isn't a Mutant - his powers are magical. Cain found a giant ruby in a lost temple, and reading its inscription aloud transformed him into "A Human Juggernaut." Thus far, there's been no indication that any types of fantasy/sci-fi elements other than extreme mutation exist in the X-Men movies. Also, the ruby is tied to a Marvel Universe demon-god character called Cyttorak (whom Marvel Studios - instead of Fox - probably holds the movie rights to as he's more closely associated with Doctor Strange.) So... probably for the best if they just make him a mutant again - just not a crappy one like last time.
Once upon a time, having a long-time good guy undergo a severe transformation that ultimately transforms them into a bad guy wasn't the eye-rolling comic-book cliché that it's become. During his most-celebrated run as author of the X-Men books, Chris Claremont poured every one of his genre-fiction fixations (love-triangles, fetishism, cosmic space-opera, all-powerful-yet-childlike-goddess-figures, you name it) into an epic four year long story-arc (1976-1980) that turned Jean Grey into an all-powerful super-psychic, then an evil all-powerful super-psychic, then dead (for a little while.) Collectively referred to as "The Dark Phoenix Saga," it set the standard (some would say unfairly) that all X-Men stories told since are judged against.
In the final moments of X2: X-Men United, the camera pans over the lake wherein the films' Jean Grey (Famke Janssen) had just drowned while saving the rest of the heroes' lives. A moment before the screen cuts to black, a faint image can be seen beneath the waves - a glowing mass of energy in the shape of a massive flaming bird. In a way, it was the prototype for the fandom-shoutout "teaser" endings that now form the backbone of the Marvel Cinematic Universe proper - it was impossible for fanboys/girls to not come out of X2 almost shaking with excitement that some version of the Phoenix Saga might rear its head in the next movie. What could possibly go wrong...
It's probably unnecessary to rehash exactly how awful X-Men 3's version of Phoenix is; though it's worth keeping in mind that a "faithful" adaptation would likely never have been possible in the first place. The totality of Claremont's original story involved a space-alien civil-war, a brainwashing mutant S&M club, trial-by-combat on the Moon, dozens of other Marvel Universe characters and Jean ultimately becoming a genocidal sun-eating space-monster; something that might not have fit with Bryan Singer's strictly earthbound X-films up to that point.