Latino Review is claiming that Marvel Studio's "other" planned 2014 release (the first being "Captain America 2" with "Iron Man 3" and "Thor 2" in 2013) - set to be officially unveiled at SDCC - is going to be "GUARDIANS OF THE GALAXY;" though it remains unclear which incarnation will appear. Where this leaves the supposedly also-imminent "Black Panther" movie (also first reported by Latino Review) is yet to be determined.
(SPOILERS FOR "THE AVENGERS" AFTER THE JUMP)
Originally a VERY-70s time-traveling space-opera series with the costume/character aesthetics of a superhero book (think "New Gods" - or "All Those Darkseid-Related Guys" if you mainly know DC stuff from the cartoons); "Guardians" has since been reimagined through multiple reincarnations/rebrandings as a kind of rotating-membership "All-Star Team" for Marvel's cosmic/outer-space/alien characters. As you might expect, ultimate-cosmic-heavy Thanos is one of their most frequent enemies, and according to the LR report that will be the case in the film as well - effectively making The Mad Titan's appearance in "Avengers" both a tease for "Avengers 2" AND this film (which will, in turn, serve as the direct lead-in for "Avengers 2."
The membership roster for the film has yet to be announced; and Marvel has a lot of cult-icon cosmic heroes to pick from - if you're hoping to see guys like Nova, Qasar, Drax The Destroyer, Adam Warlock or Mar-Vel turn up; this would be the most likely place for it. I would imagine that founding-member/frequent-leader Major Victory would have to play a key part - he's an astronaut from present-day Earth (real name: Vance Astro. For real) - time-displaced the the far-future whose "costume" is an anti-aging bodysuit, which makes him a perfect audience-POV character; plus (in a nice thematic "rhyme") he at one point carried Captain America's shield as a weapon and as a symbol (in the Marvel Universe, Cap is pretty-much worshipped as the one hero that every planet, culture, species and time-period universally admire.)
Personally, I'd hope to see more-recent Guardian fan-fave Rocket Raccoon make an appearance. He's exactly what you think he is, and don't tell me you don't want to see Robert Downey Jr. and Chris Evans react to a talking, ray-gun toting alien Raccoon.
P.S. If Thanos really is going to be the "rising big-bad" of the second wave of the Marvel Universe, the safest bet anyone can make right now is that some variation of The Infinity Gems will start popping up throughout the next few movies. We live in exciting times.
P.P.S. LR also reports that 2014 might also see the first official collaboration between Marvel Studios and Disney Animation: an animated feature-film based on "Big Hero 6;" a team of Japanese Marvel heroes (some were original to the series, some were Japanese-descended supporting-characters from across the Marvel Universe) who were part of an attempt to cash-in on the 1990s manga-boom. Something interesting to note about that: In the comics, The Silver Samurai is a member of the team... but he's also been widely-presumed to turn up as the villain of the still-cooking "Wolverine" reboot. Hm...