Variety reports that Peter Dinklage is actually up to play a key role in both of the Avengers movies, which will film back-to-back.
Peter Dinklage's role in the movies is currently unknown, and it's reported Marvel Studios wouldn't comment on the casting.
Before you get your hopes up about a possible Avengers and X-Men crossover, as Peter Dinklage played Boliver Trask in FOX’s X-Men: Days of Future Past, it's said Dinklage's The Avengers: Infinity War role is is new to the Marvel Cinematic Universe.
Regarding who Peter Dinklage could be playing, my first guess was that he's going to be the voice of a character, maybe one of the Marvel Cosmic Abstracts (Eternity?). Pip the Troll also comes to mind, which is a member of Adam Warlock's Infinity Watch from the comics. It's thought that Adam Warlock might be appearing soon in the MCU due to the large role he had in the Infinity Gauntlet comic book. In addition, his female counterpart is appearing in Guardians of the Galaxy 2 with Ayesha.
Some fan-favorite picks for Peter Dinklage to play in Avengers: Infinity War include The Watcher or even MODOK. It could also be possible that Peter Dinklage simply plays a normal human, possibly a liaison of some sort to The Avengers.
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.