Every year in the lead-up to San Diego Comic-Con, rumors and theories begin to fly around the Web about who may or may not make surprise appearances at the yearly gathering to blow everyone away and catch the ultra-informed fans completely off-guard.

But this year, a few of these surprises have leaked early or weren't ever that secret to begin with. As we prepare to head off to San Diego for MTV's coverage of Comic-Con, we've compiled a list of the surprises we already know about and the one we want to see come true.

What We Know
Nobody at Comic-Con wants to keep a secret that secret because then none of the attendees will ever hear about it. That's why you end up with several not-so-secret surprises in the days and weeks before the event. This year, the big non-surprise belongs to Metallica. The metal band is scheduled for a Hall H appearance to promote their performance-based flick, "Through the Never," which stars Dane DeHaan in the film's narrative segments, but a press release also went out about a "secret" concert weeks before Comic-Con even started.

20th Century Fox also found its "secret" Hall H plans making headlines this week, in a story first reported by Vulture. The studio behind "The Wolverine" is reportedly using the San Diego Convention Center's biggest venue to roll out at least a good chunk of the "X-Men: Days of Future Past" cast, which includes Hugh Jackman, Michael Fassbender, James McAvoy and Halle Berry. With every studio that owns Marvel characters making a play to beat that comic company at its movie game, this will be the big "X-Men" push to establish an "Avengers"-like stable of characters and actors.

On the not-so-subtle end of the spectrum, Warner Bros. and Legendary Pictures have been actively engaging fans in a viral campaign teasing a force of destruction heading for San Diego. As anyone familiar with last year's presentation and this year's schedule knows, the "Godzilla" reboot starring Aaron Taylor-Johnson and Elizabeth Olsen will have a big coming-out party at SDCC this year. Fans will also be able to participate in an event called "Godzilla Encounter" throughout the convention, but the details of that have yet to be revealed.

What We Hope
As has been the case in the last couple of years, Marvel is the panel to see at Comic-Con. Officially, we know that we can expect presentations from "Thor: The Dark World," "Captain America: The Winter Soldier" and ABC's "Agents of S.H.I.E.L.D.," but as one of the most forward-thinking studios in movies, that can't be it, right?

Marvel's other 2014 film, "Guardians of the Galaxy," just started production in London. The roles of about half the cast have yet to be made public, and there hasn't been a single photo released of the cast led by Chris Pratt in costume or even in the same room. Marvel typically sends out a start-of-production press release, but with none so far for "Guardians of the Galaxy," Marvel may have found the perfect opportunity to debut its newest big-screen heroes to the world.

Stay tuned to Comic-Con.MTV.com for more guest announcements and scheduling details. MTV News' coverage of Comic-Con begins July 18 at 2 p.m. ET and runs all weekend.