Captain America and Guardians of the Galaxy are two movies that will lead directly into Avengers 2 by this logic we can guess that both the movies will be happening around the same time or if GoTG is a future based movie then then end credits will lead them on present day Earth.
Now whatever be the story GOTG with its impressive cast has added The Collector as the villain and Benicio Del Toro who is rumored to play the role and has signed as multiple picture deal in Marvel Cinematic Universe Appearances.
We all know that The Collector has power of Prophecy, he foresaw Thanos' threat to the universe and made a massive museum to keep samples alive in case Thanos destroys the universe.
Now everyone is wondering about how to tie The Collector in MCU and especially Earth then the answer is Michael Korvac. Yes the same Korvac who was chased by the Guardians to Earth. While checking about Collector on internet I came across this Wikipedia piece that was related to Korvac Saga.
"The Collector foresaw a second major threat; Korvac a cyborg monstrosity from the 31st century of a divergent timeline, taking the form of a man named Michael in 20th century New York. The Avengers were also aware of a new threat but did not know his identity. In an attempt to protect them, the Collector added some of them to his collection and was probably planning to enlist their aid, when a second group of Avengers appeared and defeated him in combat."
This paragraph might be the key and at-least one half of Avengers will be based on Korvac Saga.
Everyone asks How will original Avengers re-unite?
The answer might be Collector himself. To defeat Korvac he captures some members of Avengers and then a second group of Avengers appear and defeat him.
Now The Collector knows that Iron Man, Captain America, Thor and Hulk are the most powerful ones right now. Throw in Black Panther and Giant Man in the bunch somewhere in this group and we get an Avengers team made by Collector. Remember that he also captured Wasp at some point of time. This can even explain how Tony got a new Armor and Arc reactor. Collector reopens the wound as part of his torture and gives him a suit of armor to play his game.
What about Quicksilver and Scarlet Witch?
Quicksilver and Scarlet Witch can either be Tony Stark's secret Avengers team or might be in Fury's new team along with Black Widow, Falcon and Hawkeye. This team will be joined by Guardians of Galaxy to don't worry about the powerhouses.
When Whedon said he wants a more personal Avengers sequel imagine what more can be personal than this storyline. I am not going with early Thanos threat because Avengers are still in their early times and Thanos will be too quick to be introduced.