4. He's A Force Ghost (Special Edition Style)
Of course, while him physically being present requires a bit of manoeuvring, Anakin appearing in the Sequel Trilogy isn't exactly impossible - he does become a Force ghost at the end of Return Of The Jedi, after all. It'd almost be a waste for him (or Obi-Wan and Yoda) to pop up at some point. The sticking point here is that the Anakin ghost would be in the Hayden form. Which is
not OK. Back in 2004 for the DVD rerelease of the Original Trilogy, George Lucas made even more changes to the movies, most egregious of which was replacing Sebastian Shaw with Christensen for the final scene. No alteration, except maybe the more catchy Han Shot First, has sparked quite as much unanimous negativity - not only is it a poorly executed change (it's just Christensen's head on Shaw's body), it doesn't actually make any sense (Lucas claimed Anakin was last good when he was Hayden, which is missing his own point) - and you can only imagine the backlash if this was canonised further by Episode VIII. The different cuts of the films raise several big questions about the true Star Wars continuity, which will only get chewed over more with the original cuts of the movies
once again rumoured to be getting a proper Blu-Ray release. One of the most important is which Force ghost Anakin is the real one? Hayden appearing would answer that unequivocally, but it wouldn't be a resolution many would be happy to see.