It's learned that¬†Benicio Del Toro will be in The Avengers: Infinity War as the Collector.
Word comes from Josh Brolin's Instagram (via Reddit) that a big Thanos scene is going to be filmed tomorrow with the members of the Guardians of the Galaxy which includes Chris Pratt and Zoe Saldana.
Josh Brolin went live on Instagram and said he is filming tomorrow with Chris Pratt, Zoe Saldana and Benicio Del Toro, who first appeared as the Collector in the first Guardians of the Galaxy movie and will also be appearing in Guardians of the Galaxy 2.
No other details about the Thanos scene are known, but Thanos does need to aquire the Infinity Stones, one of which is the Power Stone, currently being held by the Nova Corps and last seen in the first Guardians of the Galaxy movie.¬†
It did recently become known that Thanos is being treated as a main character in The Avengers: Infinity War, so anything could be possible regarding the Collector, Star-Lord and Gamora. Maybe the Collector is delivering Star-Lord and Gamora to Thanos as prisoners?
It's unknown if we'll find out much about the Guardians roles in Avengers: Infinity War from Guardians of the Galaxy 2 as director James Gunn has said there will be no Thanos or Avengers in the sequel. I suppose it's possible that the end of Guardians of the Galaxy 2 or the post-credit scene somehow leads into or involves Avengers: Infinity War, but Infinity War doesn't seem to be a big plot point in GOTG 2 (if at all).
"The Avengers: Infinity War" has a May 4, 2018 release starring Josh Brolin as Thanos. Marvel Studios has yet to officially announce the cast, but the following are thought/confirmed to be a part of the movie: Robert Downey Jr., Chris Evans, Chris Hemsworth, Mark Ruffalo, Scarlett Johansson, Jeremy Renner, Cobie Smulders, Samuel L. Jackson, Sebastian Stan, Chris Pratt, Zoe Saldana, Karen Gillan, Dave Bautista, Bradley Cooper, Vin Diesel, Paul Rudd, Elizabeth Olsen, Paul Bettany, Chadwick Boseman, Benedict Cumberbatch, Benedict Wong, Tom Holland and Brie Larson.