RONAN THE ACCUSER, YONDU AND THE CONTROLLER: A couple of sources have slowly rolled out possible villains for the film which seem to stack the odds against The Guardians in their sheer volume. JoBlo believes that Kree Empire leader Ronan The Accuser (not Thanos) will be the big baddie of the film. Having Ronan would be the first step to the Kree-Skrull War along with bringing in Ms. Marvel and The Inhumans into the MCU. Michael Rooker's Yondu will be a villainous space pirate instead of founding member of the Guardians according to HitFix. LatinoReview revealed a while back that The Controller would be a henchman to the main villain as well.
It was confirmed that both Lee Pace and new addition Karen Gillan would be the lead villains. However, it's possible they'll have a connection of sorts. While also being conflicted (at least Pace's character) about their actions throughout the film or change sides for the better. The talk is that Pace's character will eventually join the Guardians, the same isn't being said about Karen's character. HitFix mentioned that Lee could be playing Mar-Vell, Adam Warlock or Richard Rider (Nova). Gillan's character is currently unknown but will end up being leaked/announced as production is about to begin at the end of the month. I have a couple of ideas of who Lee and Karen could actually be playing.
MAGUS AND NEBULA: Magus is the evil future persona of Adam Warlock and has been a formidable cosmic villain. If Warlock is indeed in the film he would eventually factor into the storylines having connections with Thanos doesn't hurt. Speaking of connections to Thanos when it was mentioned that Yondu was going to be more of a space pirate, it sparked the idea of another villainous space pirate. Nebula is a feared space pirate captain who believes she is the granddaughter of Thanos and has clashed with the Titan and Avengers previously. It would be a way for Thanos to have influence from behind the scenes but not being the main villain.
LORD MAR-VELL/NOVA AND KO-REL: The jump to see fellow Kree supporting a villainous Ronan isn't much of a stretch. HitFix mentions Mar-Vell who has played both hero/villain and the same can be said of Ko-Rel. The Kree military leader who eventually becomes apart of the Nova Corps. might be a lock for a bridge between Nova Cops and The Kree Empire. NC has been confirmed for the film and it wouldn't be stretch to see Ko-Rel interacting with either Richard Rider/Nova or even a misguided Mar-Vell.
BLACK BOLT AND MEDUSA: Not much of information to go on with this theory besides the fact that it's more than likley that Marvel will be introducing characters from the upcoming Inhumans film in Guardians. Similar to how Iron Man/Iron Man 2 setup the entire Phase One Universe. It's possible Guardians could end up setting up most of Marvel's plans for their expansion into the cosmic side of the Marvel character catalog. Making Black Bolt and Medusa villains who eventually side with The Guardians and eventually The Avengers wouldn't be hard to imagine.
KORVAC AND CARINA: I also wouldn't be shocked to see Karen end up playing Carina Tivan (Walters), the daughter of The Collector. Ophelia Lovigood was linked as being the aide to The Collector. The Elder is best known from keeping cosmic objects out of the reach of Thanos. It's expected that Thanos won't be the villain so it's more likely like that Michael Korvac would be the threat that get's the attention of The Collector. In the Avengers cartoon series the Guardians are introduced while on the haunt for Korvac. Carina is sent by her father to seduce Korvac to find out his intentions only for her to fall in love and join him. The pair took on The Avengers and most of the cosmic powerhouses including a the trio of Odin, Zeus and Mephisto. The only problem with this one is that they would be way too powerful for the Guardians to handle on their own.