This is how I see the plot unfolding:
- Groot, Gamora, Drax and Rocket Raccoon were prisoners of the Kree. The Kree are in possession of the Soul Gem and have been for centuries. The four manage to escape, stealing the Gem and leaving Hala in a small starship.
- On Earth, pilot Peter Quill passes through a Kree wormhole, sending him out into deep space. The Kree placed the wormhole there for easy transition from Hala to Earth, as they have a spy on Earth to see if it is a threat to them.
- Quill's ship drifts through space, before it is found by the Guardians. The Kree have issued a bounty for the retrieval of the Gem and the death of the Guardians across the Universe, and quickly Thanos hears about this.
- Suddenly, everyone is after the Guardians and what they have in their care.
- Thanos allies himself with a shape-shifting race known as the Skrulls, and promises them shares power of the Soul Gem if they help him get it. He sends an armada of Skrulls after the Guardians.
- Quill is introduced to how vast and mysterious the universe is, and has to adjust to life in space, knowing he can't go home now. The team explain who they are and how they banded together years ago to fight cosmic threats, like Thanos.
- Suddenly, Adam Warlock, whom the team have met previously, appears on their ship, offering his help in protecting the Gem from Thanos. They accept.
That's all I've got that's concrete, but Magus and the Universal Church of Truth would kidnap the Guardians and take the Gem, so they have to make an uneasy alliance with Thanos (like in The Thanos Imperative) to get the Gem back and defeat the Church. Later in the movie it is revealed that the Kree and Skrulls have been at war for centuries. Adam Warlock decides the best way to keep the Gem safe is to get rid of it, and casts a spell to send it to a distant planet (at the beginning of The Avengers 2 it lands on Earth).
Halfway through the movie they name themselves 'Guardians of the Galaxy', as they are Guardians of the Infinity Gem. Adam Warlock tells Quill that he was born during a special planetary alignment and offers him the mantle of the 'Star-Lord', an interplanetary police man.
By the end of the movie Star-Lord is leading the Guardians, Thanos is ordering Skrull scientists to locate the Soul Gem, and the Kree/Skrulls re-iginite their ancient war, now over the Infinity Gem.
Avengers 2 would continue the plotline of going after the Gem, but Thanos gets it, creating a dismal ending for the movie.
I think some stuff could be cut out or toned down, but I'm certain the Kree will have some sort of heavy involvement, setting up Ms. Marvel and Mar-Vell in A2 (Mar-Vell will have been already introduced in the SHIELD show as Walter Lawson, but only reveal himself as a Kree spy in A2).