Marvel's movies are always a delicate tapestry of telling their own stories, and setting up the next ones. Take "Thor: The Dark World," which told a self-contained story that also managed to include teases for "Guardians of the Galaxy" and even a shout-out to "Captain America." But if newly released concept art is any indication, the movie could have teased something even bigger: the long-rumored, "World War Hulk" movie.

Concept artist Josh Nizzi has worked on designs for "Marvel's The Avengers," "Amazing Spider-Man," "Iron Man 3" and "The Wolverine," in addition to "Thor: The Dark World." As with most concept art, some gets used, some gets modified, and some ends up on the scrap heap. And the art even occasionally has little to nothing to do with the script, as designs are developed well before the script is finished.

Still, what you can glean from production art and character designs in particular is that someone on the production side is thinking about using those characters or settings. Whether they make the final cut or not is another question, but they don't show up out of nowhere... Which is what makes Nizzi's recently posted designs for "Thor: The Dark World" so fascinating.

While most sites have chosen to focus on Nizzi's renderings for classic Marvel superheroine Valkyrie — a literal Angel of Death who eventually becomes an Avenger and compatriot of Thor — it's Nizzi's creature designs that are far more exciting. We'll give the full run down why below, but important of note for each of the alien races is that they are designated either "Misfit" or "Marauder," which seems to suggest they would have been glimpsed in the crowd scenes following Thor's introduction in the movie. Another possibility is in the Asgardian prison halfway through the movie, and subsequent prison break.

Intro out of the way, the reason a number of these aliens are of such interest are that two of them, the Oldstrong and Kaifi (a reference to character Elloe Kaifi), first appeared in a famous comic book storyline titled "Planet Hulk." In it, the Hulk is deemed too dangerous to be on Earth, and is shot into space "forever" by Iron Man and other heroes. He lands on an alien planet and is forced to battle aliens in a "Gladiator" type situation, eventually making friends with an Oldstrong, a Kronan (the rock monster smashed to bits by Thor in "The Dark World"), and Elloe Kaifi, one of the red-skinned denizens of the planet Sakaar where Hulk is exiled.

Hulk eventually becomes Emperor (spoiler), and is finally happy. That's when the ship that brought him to Sakaar explodes, destroying the planet. Angered, Hulk takes his new friends back to Earth to punish the rest of The Avengers in a storyline titled, "World War Hulk." Guess what that story is about. Just guess.

The rest of the aliens drawn by Nizzi are all races or villains first introduced in the pages of "Thor," including arch-villain Mangog, and Grendell, a Dark Elf who plays a similar role to Algrim the Strong from "The Dark World." There's even a Korbonite, the race that eventually creates Beta Ray Bill, an alien who can wield the power of Thor and becomes a hero in his own right. But what we want to emphasize is that every design Nizzi created is from "Thor," except for these two characters, who come directly from "Hulk" comics.

It's pretty common knowledge that Marvel Studios has been pushing into the cosmic arena, first with "Thor," then with certain elements of "The Avengers," and coming up with "Guardians of the Galaxy." After Mark Ruffalo's breakout success in "The Avengers," many fans wondered why Marvel wasn't rushing a new Hulk movie into production, with rumors starting to swirl that we'd see a "Planet Hulk" movie, followed by the "World War Hulk" story in "Avengers 2."

While that's clearly not happening exactly as fans had planned, it's not too much of a stretch to think Marvel is working on a solo Hulk movie. With its post-"Avengers: Age of Ultron" plans not yet announced other than "Ant-Man," could a "Planet Hulk" movie be in the offing in Marvel's Phase 3? If these characters had shown up in "Thor: The Dark World," we would have more of a case for this potentially specious theory. But as is, it's not a clear link in the chain.

What it does mean is that someone at Marvel is thinking about these "Planet Hulk" characters. Will the end of "Age of Ultron" find Hulk at odds with the Avengers, leading to his expulsion from Earth? Or are we looking even farther into the future for this adaptation? A lot, most likely, depends on the success of "Guardians of the Galaxy" before Marvel can decided to go gung-ho into making a gladiatorial space epic. As they say, stay tuned, true believers.