Of course, most of the rumors are concerned with who is involved (there has been some speculation on whether or not Phoenix and/or Cyclops may be included). There's been speculation by MTV Geek!, which has reported that Peter Dinklage has been cast as Bolivar Trask, which would seem to lend credence to the rumor that the Sentinels will somehow be involved in the movie. Also, FirstShowing.net and others have recently reported a Bryan Singer tweet that confirms the additions of the characters Bishop and Warpath.
In keeping with the tradition of introducing more than one villain per movie, Bleeding Cool writer, Brendon Connelly, is convinced there is fire where the smoke of Apocalypse as a villain rumor is concerned. This may be supported with Marvel Comics' recent and upcoming support regarding the almost godlike villain with the short-lived Age of Apocalypse (ironically in which Apocalypse is not the central figure, but a corrupted Wolverine is) and now the announcement of the return of the Four Horsemen of Death, which shows Daken (Wolverine's son), Banshee, Sentry and Grim Reaper. Many are pointedly arguing that the name of the group shown on the cover of Uncanny Avengers #10 (due out in July) does not name Apocalypse, but is it really such a far stretch to think that the ancient mutant may be connected somehow, considering his previous incarnations of his Four Horsemen?
Let's also not forget the introduction of a child Apocalypse in the Wolverine and the X-Men series, in which everyone seems to be waiting for the child to fall back into his genetic predecessor's old X-Men killing ways. Not to mention the inclusion of the Apocalypse Twins and Apocalypse's Ship in Uncanny Avengers.
Apocalypse would be an awesome choice for a sort of meta-supervillain who can string his involvement across multiple movies which can also play into the rumors of plans already existing for an X-Men 5 movie.
-David Berck