Thanos is definitely the biggest threat both teams have encountered, so it will be interesting to see how they will merge.
Marvel started building its connected universe in 2008 with the first Iron Man movie, and has consistently released movies every year since Iron Man 2 in 2010.
As for the Guardians, it’s yet to be seen where they will stand after Guardians of the Galaxy Vol. 2 and before the big war begins.
After the events in Civil War, the Avengers roster is fractured, with its members going different ways, and with Thor and the Hulk’s fate being decided in Thor: Ragnarok.